I have an instrumental portion of a song with busy fast lead guitar riff with open low strings in Eb, a heavy tom beat on the rack drums, and a walking bass all competing down in the 80-200 hz (roughly speaking) area. The toms and bass sound fine together, but adding the guitar riff makes for blurring of that portion of the song. They work together ok for the rest of the song. Just that instrumental is very busy and blurred.
What to do ?  Any frequencies eq'd  out seems to detract from the feel too much. This must be a common problem, but I can't seem to find a compromise that works. 

carve, with extreme prejudice.
 
find, on line, a chart, that shows the frequency range of all instruments, and where they share the center freq,
shift one to the left, one to the right...
where the freqs overlap, roll them off of one, while leaving the other to take over there...
etc, etc, ad naseum, rinse and repeat,
after 10,000 hours of doing this, you will be an expert.
 
 
in other words,
there are no shortcuts,
but to sit and learn by doing.
 
get SPAN.
 
study.
listen
 
trust your mixing environment, or change/improve it.
get better monitors.
 
the list is long.......

I would echo batsbrew and add one thing:
 
After you've made some adjustments, walk away for a while. I find it's easy to feel like something is missing right after you've removed it, but you might find you don't miss it when you come back later and try to listen more objectively to the whole song.

One thing that any songwriter hates to hear (myself included) is "does the arrangement need to be like this? Could it be changed to make the parts work better together?"
 
I'm as precious about parts as the next guy - probably more so - but sometimes it's wise to take a step back when things aren't working or they're stepping over each other and reevaluate the arrangement. Rather than EQ, could you let an instrument breathe by either stopping or simplifying what's stepping on it?
 
But if the parts are absolutely set in stone, both of those replies are dead on the money. 

10-4 on all that, guys...  good advice.  The arrangement works great live, and don't want to change it... I will get the scalpel out and try further carving.  I am new at this, and yes, I can see in 10,000 hours I'll probably have it figured out. 
One other good point there batsbrew... my monitors.  when comparing my mix with other similar genre pro produced songs on my car stereo, mine are bass heavy, with some extra mud and tom resonance. This doesn't show up on my monitors well. Wondering what the least expensive pair of monitors I can get that will do a decent job. I have a great audiophile pre and power amp, so don't need powered monitors, but don't even know if they are an option nowadays... If that's the case, any suggestions for "entry level high quality" monitors ?

Check out MSpectralDynamics from MeldaProduction.  explained in great detail here:
 
http://soundbytesmag.net/spectraldynamicsdynamiceq/

Chevy
Wondering what the least expensive pair of monitors I can get that will do a decent job.

What counts as "expensive" and as "a decent job" are very mobile targets where monitors are concerned. The size of the room makes a big difference to suitable monitors as well. In a small room, for example, there's little point choosing monitors with lots of bass because the room acoustics won't allow it to develop well and there'll be problems with standing waves. In a small room monitors that roll off low bass can be better than ones that don't.

Personally I don't worry too much about sub-bass frequencies because I tend to roll off everything below a bass guitar and generally place the lowest bass around the 100Hz mark. Most domestic reproduction systems can't really handle frequencies below around 100Hz so anything lower won't be heard on those systems, and while cheap sub-woofer setups might produce the frequencies they often have terrible control over them and wallow around. If you're mixing for a club sound system of course then things are different.

Not all highly regarded monitors are full-range or flat frequency response, the infamous Yamaha NS10 being a good example. They had little bass, not much treble and lots of mids but were widely favoured at one time, presumably because if it sounded OK on an NS10 it probably sounded better than OK on anything else.

If you ask about monitors in the hardware forum you'll get lots of suggestions. Searching the hardware forum for relevant topics will get you even more. It does help to have an idea of size of room and available budget though.

One trick that might help is this. If your tracks are bass heavy compared to commercial recordings that have the kind of sound you're after import a few suitable CD tracks into Sonar along with a bounce of your final mix. Run the commercial tracks to one sub-bus and your own to another. Put a meter such as Span on each of the busses and use them to compare the eq curve of the commercial mix with that of your own, and try adjusting your track's eq until it's similar to the commercial recordings.

I'd definitely look into your listening environment first and foremost. You may not be hearing what you think you're hearing if you're getting comb filtering and phase issues from sound bouncing off of the walls. 
 
Then look into your speakers. Do commercial mixes sound OK on there? Then the problem is your mix. If they sound too boomy or too thin, then it's either your environment or your speakers. If you've dampened the room a bit so you're cutting out a lot of reflective surfaces and things still sound wrong with a commercial mix, a quick 'n' dirty fix is to strap an EQ over your output and adjust it until you get it sounding how you like it. Not a perfect solution but it'll get you most of the way there.
 
And finally your mix. One you get commercial stuff sounding good, that's the one thing left. Just bear in mind that frequencies can really build up fast when you start layering. A fat tom sound is killer, so is a fat bass sound, and so is a thick guitar sound. But together, that equals mud. Guitars especially really don't need a lot of low end when they're in a mix. They sound thin and crap solo'd but when they're in context, your ears fill in the blanks when they hear them with bass and kick drums. So be aggressive with this and then bring back what sounds like it's missing after you've solved the problem.
 
If that all fails and you really can't live without everything sounding pretty fat, then you need to turn to stuff like multiband compression, etc. but save that for a last resort. 99% of the time, good EQ will do the job perfectly fine.

Words to live and mix by:
 
Nothing is sacred.
 
Less is More.
 
 
Short explanation:  Everything in a song can be cut, edited, removed, changed... not much is carved in stone and use only what you really, really, need to use to accomplish the goal.  Then re-assess and reapply the rules previously stated.

Instruments being out of tune with each other stick out in that region as well, compounded by tuning being more difficult on lower frequencies. Another thing worth trying is using Melodyne to force pitch center on that passage, but the cutting/notching mentioned above would come high on my list too.