system: wind 7 pro 64 bit / i5 quad 2500 cpu 3.30 Ghz, 16G ram / NVIDIA Gforce GT740.
general instability:
- often in importing videos at some point it gives me clear, red or green color and the preview turns black, I only hear the audio.
- after any assembly, slowdowns during rendering.

-when I reload the sistem, if I'm lucky, I can do only few operation.

I tried to import videos in various ways: from "all media", to explore, all on one track, different tracks, nothing changes.
thanks in advance for any advice