my system used not to do this -

I have rendered a file both from timeline without pre-render, with pre-render to single avi and from pre-render timeline - and where effects are applied or transitions it just freaks out and will drop text or flash all crazy, thing is it's not consistant - sometimes it will happen in one place then next render it will happen another, I turn of supersample and it's fine (but slowmo looks like crap

I;ve defragged, cleaned up pre-render, etc - I dont understand - please help!