Hi Everyone,
 
I just upgraded to X2 this weekend, which involved adding memory to my motherboard and loading Windows 7 64bit OS.  I'm using a Yamaha XF8 connected through the Firewire interface to the computer motherboard.  I loaded the Yamaha Steinberg Driver for the FW card, and the MIDI ports for the XF8 show up where they are supposed to be in the Sonar Midi devices. 
 
I did a test recording of simple eighth notes at 100 bpm, then played it back.  The playback tempo is steady for a few measures, then pauses for maybe a 16th of a beat, then continues.  This happens every few measures.
 
The problem seems to be with play back--when I record notes at tempo, it's right on the beat. 
 
Any help appreciated.
thanks,
Steve

You may have quantize set to 1/16th notes. Check that.

Another thing to check is the midi buffer setting in preferences. Hit the P key and go to midi playback. You can try increasing the buffer size till you find a setting that works.