XPS 8100, half-shutdowns, random

I'm having an odd issue with my 8100. I realize it's an older computer, so that could be the simple answer. But, I thought perhaps I would check to see if anyone has experienced this issue before. I have an issue where my pc shuts down randomly (sort of). What happens is my monitor will turn off, but the tower will stay powered on. I try to turn the monitor back on, but it just gives a black screen, then the monotor power light goes out again. I have to hold down the power button to shut it off at this point. I think it's not (just) the monitor, as one time this happened while I was playing an online game, and running a voice chat program (Mumble) - and I was immediately disconnected from both when the monitor failed like this. MY guess is if it was simply the monitor, I wouldn't have lost my connection to hte online game and voice server. Also, I'm not sure if this is related, but I also have an issue where sometimes I will log on, and I will get the "Windows failed to connect to the System Event Notification Service", and I have to reboot a few times until it goes away. Ususally, this seems to occur directly after the monitor thing happens, and I have to force a shut down by holding down the power button (described just above). However, it has happened on its own as well - so I am not sure if it's related. As far as specs, the pc is the 8100 XPS, first gen i7 870, 8gb ram, Radeon 5770 card, Windows 7 Home Premium 64 bit. I thought perhaps it was a heat issue, and it still may be, but I'm not really that knowledgeable to diagnose if that's the problem. I did pull the case off and dust it off, but that didn't seem to help. I think the fan may not be working on the card, but I'm not 100% - again I don't know if the thing should always be on or not (it isn't ). Can anyone provide any assistance or insight?

Update - downloaded HWmonitor, when I am playing a game (SWTOR), all my cores are running over 72 C. I'm not the smartest guy with this stuff, but I think that is insanely hot? I have 2 fans, I messaed around with the inside of the case, I have nowhere I can put any more. Any help? Is the heat my problem most likely? 

Thank you !