Drop of sound quality

Lately I have been havinga quite annoying problem with the playback sound in Cubase Essential 5. Everything is normal until suddenly I notice a slight switch in the panning of the instrument I am using, simultaneously I experience a loss of the “bass quality” of the sound. Very irritating when playing a cello, for instance! Never had this before. I get the normal sound back by rebooting CE, but after some minutes it is back again.

Now, where should I start to look for the problem? I am using an internal Audiophile 24/96 with asio drivers and all that. I would suspect the headphones or headamp, but I can’t notice any problem with audio-files (and why would reloading CE help?).

So far, it seems like the problem only occurs when I am using Garritan Personal Orchestra with the Arial sampler as VST plugin. Could it be a problem with Arial??

Other system specs: XP SP3 and a Intel Core 2 Duo E6750 @ 2.66, 2.67 Ghz.