Playback intermittently started to skip

I've almost completed a session in Pro Tools 9 with more tracks than I've ever recorded successfully. Kick,Snare & Hi Hat stereo instrument tracks, converted to audio tracks with ezdrummer 2, and 17 other mono tracks including aux. tracks and loaded with plugins! A double tracked lead vocal and a 3 track vocal harmony background! I knew it was too good to be true! I was starting my mix when intermittently the playback started to skip! It sounds exactly like a vinyl record when it skips over tracks! It never happens in the same place twice! This has never happened before and I'm at a loss as to the cause. What could be a logical reason for this happening ? :eek: