Okay, that already sound much more sensible. From my best guess it could be either of those (in no particular order)
- Faulty graphics card
- Overheating graphics card
- Overheating CPU*
- Faulty memory
- Faulty or too low powered PSU*
* I think these are most likely
I'd first check the case for dust and if all the fans are seated correctly and working, then if everything looks OK install some sort of tool that shows you the CPU, GFX card, etc. temperature (there are plenty out there, some freeware/shareware product ought to do it). The best would be if it has a temperature logging function because in non 3D mode it's likely that it's not overheating at all. So fire up the game and when you feel it's shortly before breakdown Alt-Tab out of it and see what the temperature readings look like. Be advised that temperature readings can rise and fall very quickly when you enter/exit a game, that's why the logging would be advisable. If they are above 70°C for the CPU or above 90°C for the GFX card then you're in trouble and need new/better coolers. Below that should be fine, or at least not causing the sudden shutdowns.
That said, overheating of the GFX card normally results in plenty of artefacts before the system shuts down completely, whereas CPU overheating can cause everything to break down relatively quickly, so IMHO if it's overheating, then it's more likely the CPU than the GFX card.
The next most likely thing is that the PSU is faulty or simply not able to provide enough power to the system. Unfortunately I can't give more specific details on how much power you need or how to resolve this (other than trial and error with a better PSU), as I don't know what PSU you currently have nor what your CPU & GFX card combo actually need.