Russell i's the switch as usual. The contacts just go bad, quite normal in DC switches.
OEM is C&K Switch 1101M2S3CBE2. That's a silver plated contact....I do believe there are gold plated contact versions available (which should last longer) but I haven't seen any of these.
Best just replace with the OEM:
RS Components: 161-7640 $6.41 or E14: 1437698 $9.44 + $post There are cheaper switches that would fit....but...IMHO...use to correct one. If you solder it in....Keep heat on the pins to a minimum as heat allows contaminates to form on the contacts.
I have one new in my spares. Same switch was used in some earlier Gemini 1's. If you want me to replace it I can as I'm keen to get a copy of the EEPROM firmware.
I have all parts for the 492 system except the firmware.