Check circuit board for any dry solder joints (re-flow solder with soldering iron if necessary). Maybe also clean switch with switch lubricant. Unfortunately this didn't fix my fault.

My final repair attempt, was to heat up the circuit board in a fan oven to heal any invisible microcracks in the solder. I started at 5 mins at 120C and went up in 10C steps, testing after each step. After 160C, the keyboard was working much better, but after 170C it was dead!

Hopefully your repair will be more successful.