As an initial step I would suggest checking that the QTG test 1.b.7.1 still passes correctly. Hopefully the MQTG is available for comparison.
That should at least tell you whether the sim is still performing to its original standard.
By inference, a classic sim is probably 15+ years old and is likely to have been modified quite significantly since initial build by either the manufacturer or operator(s) and errors can and do creep in.
Edited to state that Old Smokey's post seems to indicate that this is a -300 that was, I believe updated by the original manufacturer quite recently. I would have thought that the manufacturer would have performed a complete QTG run as part of their acceptance process. Should make it a lot easier to determine when the problem arose and if it's a data/software issue or a maintanance issue.