Retro Replay Contact Problems

Contact problems are the one major reason for strange behavior observed with the Retro Replay. Before even suspecting anything else ensure to have proper contact. :)

The Problem

The Retro Replay itself has gold plated contacts, many if not most c64s however have tin, brass or other cheap metal contacts at the expansion port. This results in increased oxidation at the contacts (an effect well known in electrochemistry).
Additionally the cartridge board is 0,2mm thinner than it should be

Fixing

Clean the contacts of both the cartridge and the expansion port. You may use a rubber or sand paper for that.
Bend the contacts of the expansion port so the cartridge fits more tightly.

Ultimately:

Put some solder onto the contacts of the Retro Replay itself, which makes them thicker and also "softer" - resulting in better contact. (You may think about filing off some of the gold from the contacts - and possibly sell it.)