are you sure, that what your screen displays, deviates among 30m from reality? in my case, the difference is about 5 to 10m and it recalibrates itself, when i turn left or right on a crossroad.

you wrote, that you upgradet from MKII to MKIV. perhaps the gps-reciever is "too old" and it recieves only a fixed amount of signals.
the accuracy depends on your environment, so it should be less, when you're riding downtown, because of the skyscrapers, or through a tunnel.