Has anyone come across a problem like this!!

Using sounds like "Heavys Trip" + "Monster Lead" I get an intermittent problem where if I play a note the pitch does not stay the same (it will waver between what it should be and a semitone lower). I'll be halfway through a solo and I'll find myself 3 notes lower in pitch!!! Very frustrating and I've run out of ideas.

I'm wondering whether there is a fault with my workstation.