Yes, the problem may lies in the flex cable ribbon, if not , then the screen has some problems.

It sounds like the ribbon cable that connects the screen and the pcb is damaged or there is some dirty in-between the contacts of the slider mechanism. The only way around this is to take it apart and find out. I recommend you take it to AT&T to be serviced.

I've read of others with the same, or opposite problem (screen goes black when the open it).

I have had my samsung impression for quite a while now with no problems. Lately, though, my lcd screen has been having some problems. It works fine when I have the slider keyboard out but right when I slide the keyboard back into the phone the lcd screen goes black. It used to stay lit and show my background and everything. Is there a way I can fix this? I have tried taking the battery out and all that but that doesn't seem to work either. Please help!