Poor contact between thermal pad and chips on my VGA card

[lunix]

I recent purchased a Aopen Aeolus 6600 PCIe 256MB video card. No fans, just a heatsink. Temperature wise it holds up pretty okay. It doesn't have an internal thermocouple so I attached an external one to heatsink and measured it at 65C in the worst case running the WoW. However once in a while my WoW will crash but I haven't been able to predict what is causing it. I was thinking it was my video card because screen goes black and switches between desktop and black screen.

To make a long story short, I examined the contact between thermal pads and the RAM chips and it is very poor and I can see a gap. I can squeeze the heatsink to the PCB to maintain the contact but later it forms a gap again. It is hard to tell if GPU has the same gap problem or not. So I am wondering if this might be the source of the crash problems?

So I am thinking if I should exchange the card for a new one hoping the contact issue if fixed on the other one or should I remove the stock heatsink and replace with Zalman VGA cooler?