Ok, well heres the deal. My original intent was to not SLI or crossfire anything, therefore I purchased the HD 3870x2 because I heard it was just two 3870 gpu's glued together and worked like a happy family should. So, if I'm reading what you typed correctly, the HD2900 serious required a crossfire board to use both gpu's on a single card? So in that case, you're wondering if the 3870x2 uses the same process, and in that case I know for a fact I have an SLI compatible mobo (not crossfire) hmm. So, it's possible that getting a new motherboard may be the trick? Also, my current card (ironically) IS a x1950 pro (by sapphire) one of the best cards I've had, worked like a charm and plays games better than I thought it would. However, this card is also a crossfire supported card, it came with the crossfire bridge and I even still have the box. BUT like you said, the 2900 series began requiring a crossfire motherboard. So although my previous video card, mr x1950 worked, its apparent that maybe the 3870x2's are still primitive to the new 4870's in the manner that they require the crossfire mobo.
After all that pondering, I really hope not...... but I suppose snooping around the internet never hurt.
The monitor works fine, I put back in my x1950 every time the 3870x2 doesn't work (so my back hurts pretty badly now cause I get to put it on the floor in a corner. So therefore, the monitor works every time I plug it into the x1950, and ceases to display with the 3870. Also, I suppose the 2900's didnt support non crossfire mobo's, hopefully is not the case with the 3870's, but I suppose I'll look into that it seems very important.
My case is an antec 900, its got plenty of fans as well, I can't imagine first of all that the card is overheating that quickly to even display anything, or that it's even overheating in the first place. Even though my pc is in a corner, I clean off the dust from the fans and the thing has never had any problems with airflow.