Do you get a flashing question mark in the VFD window (the little florescent window on the console) when this happens? It will be on the right side of the window. If you are, it means you are losing clock. What is your clocking setup?
Well I had this happen to me today during a session with a client will broadcasting to our site live so it was quite embarrassing. I went to the back of the console finally and reseated the clock card and now it seems to be working fine now. Hopefully that was the end of that because it cost me a half hour of down time from recording today.....