I plan to take this to the LBS in the morning if I don't find a solution tonight, so I thought I'd ask you all:
The cadence function works just fine, though once in a blue moon it will drop down to 30 or 40 rpms inexpicably. This is rare though.
The speedo, on the other hand, is out of whack. When I am pedalling (usually around 90-110 rpm), the computer will show a very low speed like 4-6 mph. As soon as I stop pedalling, and coast for a second or so, the computer will register actual speed. It then goes back down as soon as I resume. Oddly enough, it also registers correct speed when I change cassette gears. The odometer also reflects this problem. When I've gone say 15 miles or so, it will only register a small fraction, say 5 miles.