I had a somewhat similar problem recently, where some instruments would get dropouts at unpredictable spots. In my case, it turned out that there was a Z3ta 2 update since the last time i had worked on the project, so all of my Z3ta 2 synths were acting up until I updated it (purchased through Steam, here). If you don't have z3ta 2, or if any of your unruly synths aren't z3ta 2, then it's probably not your issue. Hope you solve your issues, can't stand wanting to work and being stymied by technical issues!