Another potential problem may be that the RAM you are now using could be defective. Have you tried to replace the ram with entirely new sticks to see if that is the cause~?

If you don't have any additional RAM sticks available, you could try swtiching the RAM so stick 1 is in the second slot and vice versa. If the computer seems to lock up more quickly than usual, the stick you moved to the first slot may be the culprit. This is by no means a truly scientific test but just a means by which you can guage the trouble a little bit.