Alright re-soldered the interconnect (it already had the new square pins installed), and the machine has not reset since .
Also, found a loose wire/pin on one of the connectors which caused the problem with the advance button, so that now works.
I'll rebuild the flippers and all should be sweet with the machine. Thanks Strangeways and ddtoys for the help.

I found a great website which has detailed info and pictures on how to repair all Williams machines from '77 to '83 so i'm pretty happy right now.