I really appreciate the responses! I'll try cleaning the controller first. If that doesn't work, I'll search for an Intellivision Flashback and buy the adapter.
Be VERY careful if you open the controller to attempt to clean the mylar. The printed traces are very fragile, and rubbing too hard with isopropyl alcohol can take sections of the traces off the mylar.
More than likely, the left-right area of the traces are worn on the mylar inset, and replacing it will be the best option. The hand controllers are very easy to disassemble, just 4 screws on the back of each, but when you take the front off be careful to note which way everything goes back together, including how the mylar is folded.
I tried conductive pen to attempt to repair traces on an Atari Lynx mylar controller insert, but had no luck. Anyone actually tried this on an Intellivision mylar insert with positive result?
Edited by HunterZero, Sun Sep 10, 2017 7:44 PM.