Hi M2M, I think this sort of problem was discussed some time earlier, and the reply was that it is likely to be an encoding problem. The code used is Grey Code, which changes only one bit at a time, which is different to Binary Code. It could be just one wire connection loose... So try rattling each connector, to get a better joint.
Does the fault only occur at those two heights +- 50ft ?
Or is it continuously wrong ?
and I take it you mean Flight Levels.