You should have AMD Catalyst Control Center installed. If not, download this and run it. It'll scan your system and recommend the best software for you. Once yu've got Catalyst installed, run it and click on "AMD Overdrive". You should get a warning about overclocking, accept it and go further. We're not everclocking at this point, merely using the temp monitoring tools (although you can overclock with this software, it's very safe, won't let you come near voltages or anything). Check out your temps, you should be idling at 40ish.
Once you've confirmed that, download this and install it. The aim here is to benchmark your graphics card and keep an eye on the temps. If you don't go over 90, it's all good. If you do, that's where the problem lies, although I highly doubt it.
To be honest it happens from time to time that a PC makes you go "huh?". if it's a one off I wouldn't worry about it. If it happens more often, it might be time to look at possible causes.