I get that too often, too! I can only offer my novice workarounds:
1. It seems to occur worst when I insert certain effects - notably the ones I wanted X1 for in the first place (e.g. LP64_EQ_64, etc.) - so I either don't use the respective effect (however, the problem sometimes seems to remain even after deleting the 'offender' from the effects bin, so knowing in advance which effects not to use seems to be the bet). Or, I bounce the MIDI to audio and then use the effect.
2. As the CakewalkSoundCenter soft synth doesn't seem to randomly drop hits, I'm resorting to using that instead of Session Drummer
... all of this makes me angry that I spent the money on X1! I suppose a patch would solve the problems?