If you want a reliable System 6 board set, you really need to do the following:

1) Reflow solder or better yet replace all header pins on all connectors (this includes the sound and power supply boards)
2) Replace all the ROM, CPU and other sockets that may be on the boards
3) Clean the pins on the CPU and ROMs
4) Replace the 40 pin interconnect (both sides)

There are some other enhancements you can perform but those are optional and don't effect the reliable operation of the machine.

If this seems like too much work or too much money to spend on your original boards, sell them to me and buy a new one.