Uneven swing feel on playback of old .wrk files

I had some time to do some more experimentation to try and figure out my problem.
It seems that it may have something to do with tempo.  I examined one of my projects that played back correctly alongside one that didn't and noted that the first was set to 150bpm and the second to 170bpm.  By altering the second track's bpm to 150, it too was able to be played bacl correctly, albeit at the wrong tempo.  Curiously though, I have another project with a bpm of only 140 that plays back incorrectly.  However I could make it play back with an even feel by lowering the bpm as well.  As far as I can tell, every track I have will play back evenly at 100bpm, and the higher I go above that, the more tracks will start playing back with an uneven feel. 
I thought it was a swing problem initially, but it doesn't just happen with eighth notes, but I'm not sure how to lable the loping, uneven feel the incorrect playback produces.
Additionally, I tried creating new tracks and found that a new track with 170bpm with be played back incorrectly.  I did not have time yet to try every blm and determine where the exact line is. 
TLDR: I have refined my understanding of my problem to - tracks play back with uneven feel above certain tempos.
Any idea what could be causing this?  Is it a memory or buffering problem maybe?  I'm still stumped.

scook
...The X series default is 250. Prior to the X series the default was 500.

The 250 default only applies to the 64-bit edition. It's still 500 in 32-bit.

Might want to provide more information about the system and the projects than just the behavior.

A few possibilities spring to mind....
 
MIDI buffers still set too low. Try 1,000.
 
PCI bus interrupt requests interfering with Sonar/Windows ability to stream audio in real time. Things  like wireless network adaptors are often the offender if that's a problem. There are downloadable free tools that can check for this, such as LatencyMon. There's also this one which for a long time was the most recommended tool and gives far easier to interpret results, but according to the developer it isn't suitable for use with Windows 8.
 
Could a dodgy MIDI clock source perhaps cause the symptoms defined?
 
Out of interest, does the metronome (settings are in the per-project part of preferences) play in time using both audio and MIDI outputs?
 
What happens if you bounce the MIDI track to convert it to audio? Do the audio transients line up with the grid correctly or are they displaced? If so, by what kind of amount? Same question if you "freeze" the synth.
 
Finally, details of the PC, Windows version, whether 32 or 64 bit Windows/Sonar and audio/MIDI interface might well be helpful to people trying to assist you.
 
And apologies in advance if you reply to me and I don't get back to you quickly. Going away for the weekend tomorrow and I don't know how good (or, most likely, poor to non-existant) the G3 mobile internet coverage is going to be.