Freezes and Display Warnings

I bought my PC half way through this year it has been running great and still does except lately its been having errors usually whilst playing games like World of Warcraft. "Display driver amdkmdap stopped responding and has successfully recovered" Event ID 4101 and it also has been completely freezing where I can't do anything so I have to restart the computer though when this happens I can still hear my music playing on iTunes.

I have just last week done a fresh install of my Windows 7 64 bit Home Premium and have also made sure that there isn't dust on my fans. I contacted iBuyPower.com.au where i had bought my pc from and they believed it to be an overheating on my GPU and I have monitored the temperature of it whilst in game on ATI Overdrive and it generally averages around 75c fan speed at 100%. All drivers are up to date latest Catalyst Control Center.

I and some friends believe it to be a PSU problem because it is only a 550W but I am unsure so I decided to post here.

Thanks for any help anyone can give - Paul

The PSU might not be the culprit, the HD5850 needs at least 500W

This is a TDR error usually caused by overheating,Bad RAM/GPU
Test your RAM http://www.sevenforums.com/tutorials/105647-ram-test-memtest86.html
It seems that the GPU is faulty consider getting a RMA