Vari-Audio dropouts

Had a wierd one last night. I was mixing a song which initially included a single vocal track. I did some minor pitch correcting with Vari-audio, then made 2 copies. One manually changed slightly to act as a double for certain phrases, and the other pitch changed to create a harmony part. After completing the parts, and insering a buss compressor o the vocal group, I started getting intermittant drop-outs on the various individual vocal tracks, both in playback and on export. The entire sub wasn’t dropping out, but rather an individual part (e.g., the original, the doubled or the harmony would drop for a few seconds while the others continued to play normally) It continued even when I removed the compressor from the group. I tried to mix to MP3 and got lots of vocal dropouts. 4 passes rendered different numbers of dropouts in different sections. I finally tried a real time export in WAV (note I’m not using any external devices) which rendered a usable mix. (I then converted it to MP3 in Audacity)
Any idea what could be causing this? :question: