Here's a idea, i had the same problem as you, do you have a metal case? if so i susspect that when u swapped cards you put pressure on the mobo which in turn lowered it so slightly you would never know but low enough to make one of the little metal spiky bits ( not a technical term i know ) make contact with your case, apparently there was a longer one on it than there is meant to be, maybe a design flaw. As the case is metal BAM you get a short circuit and no signal. This happened to me and it took frikkin ages to find out what the prob was, the guy in the shop spent ages looking and testing only by chance did he spot it. He just gave it a quick file down ( be careful) and it worked. I hope this helps and you can sort it out
