it's random. Guru had the same issue. Angus had told me it would be fixed in an update but the workaround for the time being was to change the number of processors in Protools to 1. That was about 2 years ago.
i had flamming problems with guru, and now again with geist. one thing i noticed was that guru flammed with protools HD, but not protools M-audio-powered. i had guru running ok on HD by the end of it (not perfect, mind, while timing was tight under m-audio but loose as hell in HD, plus occasional flamming when tdm was mixed in, but it was still usable) but now that i've switched to geist it's buggy to the point of being unusable. frustrated! i've got to make 50-odd beats soon for work, which would be a great jump off for me getting into the swing of using geist, but so far geist's just been too buggy to use.
I have confirmed this in my PT9 HD setup as well. As a test, I created a click track using a clave sample in Geist, recorded it to an audio track, then put a phase reverse on the audio playback track. When played together with the live Geist, the audio track will go in and out of phase (ie sound and then not sound), and never repeats the phasing on the same beats. I noticed this problem at first because I went to record some beats for mixing, and some of the downbeats were as much as 154 samples late.
This happens whether delay compensation is turned on or not! I wil try messing with buffer settings but currently I am at 256.
I don't know it this helps, but I had some issues with sync and PT9. Keep on ADC and choose "short" from the playback buffer size. (Separate from the ordinary latency setting). That did the trick for me for the sync issues. Just a thought.
yes, getting the playback buffer size down low is what fixed things for me (as explained in a pm from the supposrt staff at fxpansion, thanks again guys!) it seems counterintuitive (shouldn't big buffers help with virtual instruments?) but that seemed to do the trick.