I assume you've got all the patches for the software? The behavior you describe is eerily suspicious of some problems that Joe fixed in the code last year where the serial port was receiving incomplete packets but the code wasn't detecting it correctly. If you don't have all of the patches, I'd recommend checking out
this post. If you do have all the patches, the Mac may have uncovered additional problems in the serial code that we have yet to debug/fix... Let me know which case is more likely and I'll see if Joe has some time to look into the problem.
-Danny