Well, so you don't have to wait for tacit...
If you have a lot of RAM and you are using other than X, hold down the Command + Option keys and then select the Memory Control Panel. At the bottom is a radio button to turn on/off the memory test. If it is on, turn off. If it is off, well, then we look elsewhere...
A PRAM zap can reset the memory test to ON, BTW as that is the default.