It might also be a hardware problem. I once had all kinds of similar weirdnesses that eventually turned out to be a simple hardware issue. The entire case story would be too long to tell, but the upshot of it is just this: you wouldn't believe what weird effects a slowly dying power supply with the occasional associated bit errors in RAM may produce...  (and our minds are very good at trying to put meaning into the (sometimes random) coincidences we observe :)