Firstly, is the PSU up to the loads you are putting on it? try running games at lower resolution and 16bit colour and shut down any other progs before running games. Try leaving the side off the tower (if that is what you have) and if it is in a restricted environment move it out into the open. Sometimes you may need to remove the heatsink on the CPU and clean the die and reapply some heatsink compound. A 550Mhz processor is a bit low for todays games.