Sonar's 64bit engine stealing my bass

Tried to mix down a song in Sonar Platinum today.
After the tweaking was done I did a mixdown, and to get maximum soundquality I enabeled  the 64bit engine in the export window. I used audible bounce so I could hear what was going on.
After a couple of bars I heard that something was wrong. I sounded like the bassguitar was fading down and almost disapeared in the  mix. I stopped the bounce process and checked if there was some level automation that was wrong, but the bassguitar had no automation. 
I tried a new bounce but the same thing happened. I stopped and checked if there was some compressor settings that fooled me, but I cuoldn't find anything. The song played fine when I was playing it in Sonar, but as soon I tried to export, this strange thing happened.
Then I unchecked the 64bit engine and the export/bounce sounded exactly how it was supposed to.
 
Anyone else with similar experience?
 
Edit: I'm using the latest update

Is the bass guitar something you tracked or is it from a sample library or softsynth?  If it's from a sample library or softsynth, maybe it doesn't play nicely with the 64 bit engine.  Maybe you'll get better results if you freeze the track first before mixdown.

I had a similar thing happen to me. Only it seems to be a different track with each bounce. Snare gone, then the guitar is louder for no reason. In have spent days working to isolate what is going on. My solution is to set up a second computer and just dub it off like the old 2 track days. The mix plays fine but bounce is just not usable or trustworthy. I had similar problems back with 8.5.  Who knows what it is... converter drivers the DAW itself ( gasp I said it) or something else. With only forum help to go by, there is no accountability so..... welcome to the soul sucking repetition of the new home recording world.

For forum help to be of any use, we need data. "Spent days working to isolate what is going on" is the only data we have, and it's a possibility that spending days trying to isolate things without a strategy may have created all kinds of weird variations from the defaults. We don't know what version of SONAR you have, like whether you have the one with useful presets in the export menu. 
 
I bounce everything from individual tracks to entire compositions for a final two-track mix with no problems, however for you to arrive at that same place we need to know your interface, the drivers you're using, what your export menu settings are, etc. Bouncing is a simple and trouble-free operation but it's a combination lock...if each tumbler isn't set correctly, the lock won't open.
 
Hmmm...maybe "Taming the Wild Export/Bounce Menu" should be a future "Friday's Tip of the Week."

As for the bounce settings I have tried with and without the 64 bit engine. No change. I ran tests with fast, regular and audible. No consistant repeatable bounce was achieved. latest drivers on the motu. Its a real disappointment. 

JonD
Probably better if you started your own thread, and provide your system specs and other pertinent details.

E.g., a screen shot of your export settings.