I've had to replace two wire harnesses. The little retaining tab on the mount where the computer slides into the bracket broke twice. They say they've redesigned it so it doesn't break, and indeed, mine hasn't broken in more than a year now. If the computer wiggles in the mount, and slides out too easily, this could be your problem--though it sounds like this isn't your problem. But one symptom of a broken mount can be intermittent contact.

I like the computer otherwise. I have my magnet a bit further away from the sensor than recommended, and it seems to work fine.

I hope it lasts 20 years.

Try new wires, yeah.