If you had the unit for a year or so, you might also check the battery.

There are several things that can give you a bad reading.

1. As DebW pointed out, magnet has to be pretty close to the pickup sensor.
2. As li10up pointed out, interference from other wireless computers and HRM.
3. a weak battery
4. bad battery connection or a dirty connection.
5. a damaged computer

first one is easy to check, the second one go out and ride on your own to see if the reading is normal or not.

If it isn't the first two check the battery and the battery connection inside.