Third Mem slot of BP6 often gives this problem - especially if all three modules don't match. Some have solved this by shuffling thru all position arrangements until they hit one that works. I'd be looking to sell my two 128MB modules & get a 256MB if i was in your shoes;
Living with the BP6 gets you very intimate with some of the more strange & quirky issues of PCs. Having this website available & active makes all the difference when you're stuck in the middle of one of these senarios!!