I too have this problem, only noticed it after i updated to the latest Avermedia software a couple of weeks ago. I am sure it was the update to the latest software. Everything had been fine before, but immediately after i installed the new software the camera images would jitter, maybe only every minute or so, but very annoying as it sets off the motion detection.
Interestingly i had the same problem when i first got my NV3000 a year ago. I added cameras one by one as i purchased them over a few weeks. With just one camera installed I had this same intermitent jitter. When i connected a second camera the jitters stopped. I added a third and they started again, then a fourth and all OK again and has been ever since, until that is i updated to the latest software
incidentally i have XP p4 3.2GHz ATI Radeon X300SE 128MB