I have a A800
24 here and normally it is the molex connectors. For a temp fix is to re-seat the cards on the channel you are having trouble with.
I had this problem for years and finally had enough and my tech and I spent 2 full days cleaning/tightening the molex pins and sockets.
It wore my fingers out, put a crap load of little cuts in them but I have not had a channel drop since. (We used pipe cleaners) and cleaning solution.