Some further testing:
Memory
I tried to use only one of the two 1gb dual channel memories in every slot (out of 4). Lag is still there. Tried the other memory in all slots, but the lag i still there. Tried the memory pair in different slots, but the lag is still there.
Conclusion: it's not memory related.
Processor
I downloaded SpeedFan to monitor the temperature of the processor. Temp is constantly between 30-32 degrees.
Conclusion: it's not an overworked processor causing the lag.
Video
I tried to play a DVD-movie. Strangely enough the lag disappeared for most of the time, but I could still hear small lags here and there. Video did not seem to be affected by the lag, even when the sound lagged. When I played music after closing the movie it seemed fine for a long time (at least 10 minutes, then I had to go home from work). Until I came back today and started winamp, the lag is still there
Conclusion: The lag seems mostly related to sound. I don't rule out that it affects video though, I have a feeling things just "decided" to work smoothly for a while.
Well, I still have a feeling someone has a miracle advice here, I'm just waiting for it
