For anyone who wants to really get to the root of this problem you can use the "bounce" test ROM in this package: http://pincoder.ca

It will easily show you if any of your switches are in fact double bouncing. I wrote this ROM because I have exactly the same issue with my gorgar. and yes, it's occasional.

When I run the bounce test on mine, the switches register exactly as they should, so for my gorgar, the boards are fine, as are the switches and wiring. I can only assume the problem is in how Williams wrote the GREEN ROMs.

Having said that, if you run the bounce test ROM and it shows actual bouncing on any switch then you should start with the switches and make sure they have the correct gap etc. Then look at wiring. If all is well there then the problem is likely a bad PIA chip. The documentation for the each of the test ROMS will help you find the problem.

There are many other valuable test ROMs you can use to check the rest of your machine. Download a copy and take a look at the docs for each test. They're very easy to use!