Hi TDB,
I am a bit surprised that you did not contact me first about this issue. Oh well.

The issue is with the Tape/Input switch, not the Stereo/Mono switch.
There are several ways to deal with this issue.
First, get a can of Caig Labs D5 Deoxit spray cleaner and use that on the switches along with cycling them all 50 times or so. Spray the switches from the inside of the chassis, from back to front, into the switch lug areas. The cleaner will get into the contacts. You may need to wipe the faceplate area clean a few times to remove the excess that leaks through. The cleaner won't hurt anything.
If that does not clear the problem there are three more things to try.
First, if you are handly with a solder pencil, you can install a small jumper wire on each vertical half of the Tape/Input switch between the middle and bottom lugs on each side, leaving all the original connections in place. This takes the switch out of circuit.
Second, you can simpy ask us for a free replacement switch if you feel comfortable replacing it yourself. Since the connections on most models were done with push on connectors, it is a pretty easy project.
Third, you can send the unit to us to fix it free of charge except for return shipping. $35 in the USA.
Please call me at 651-330-9871 for more help.
Frank Van Alstine
PS The problem arrived only after these units were in service for many years and then only with units where the Tape/Input switch was never used. The switch is designed to be self cleaning each time it is used but evidently some owners never use this function and then oxide build up occurs.