My Mac Pro 2,1 has issues with watching videos online. I have 8G of ram and an Ati 5770 installed. These issues started a month ago, then seemed to go away. After an Apple software update a few weeks ago, the issue returned and won't go away. I installed SMC Fancontrol, which seems to help buy a few minutes of netflix runtime, but not much. I've run Apple Hardware Diag., and we passed all tests. Out of frustration I pulled my old hd out of the closet, with the original 10.4 install on it, and everything was fine for an hour. However, we can't watch netflix with Tiger, and I wasn't able to get full screen video with anything else online (YouTube, Amazon, CBS). I think it's safe to assume that Tiger stresses the hardware less than Leopard or Lion. I also tried isolating Dimm modules to see if was a memory issue. They all seem to work alike. Pulling one of the riser cards out keeps the machine from restarts, but either one will work, in either slot. It seems like I have symptoms that point in both hardware and software directions. How can this be?