Those temps you mentioned - are they the max you see? They seem reasonable under load. I am assuming that you have active (air or water) cooling on both the cpu and gpu, rather than passive cooling. Some ideas:

1. Dust is the first obvious culprit - make sure your heatsinks are all clear of dust bunnies!

2. Run under load with the sides off your case. If you are still getting artifacts or crashing issues, it may be a heatsink problem on the GPU or CPU.

3. Consider removing all unecessary hardware to see if the problem persists, re-attaching each device until the problem returns

4. Check your case fan setup. If you have more intake fans than exhaust fans (aka positive pressure setup), you may be having a problem with turbulence in the case, which causes nasty hotspots. This can be solved by reversing enough intake fans such that you have more exhaust fans than intake fans, a setup known as 'negative pressure'. I use this setup myself and can recommend it against the positive pressure setup.

HTH