I tried the new keypad with the additional grounding layer and it made no difference.

Do you suspect an EMI issue? If so, is there anything else that can be done to the display/keypad to reduce the effects, while I try to find the source and reduce its emissions?

The back of the membrane keypad is adhered to a grounded surface, but the keypad itself does not include grounded top layer. The display is mounted under the keypad in a cutout in the metal housing, just big enough for the display, which shows through a clear section of the keypad. I ordered an ...

After many hours trying to come up with a repeatable scenario of failure, the keypad still hangs when certains keys are pressed.
The hang is intermittent and not repeatable without the keypad connected ( i.e. shorting rows to columns does not cause the failure, at least not in an amount of time ...

Yes, I am using a plastic membrane keypad. Are you suggesting to short a row to a column to simulate the offending keys, or are you suggesting to short two or more columns or rows to each other?

The PCB rev is 1.23, and the CPU has sticker on it that says V1.8, I assume this is the firmware version.

In addition there is another sticker on the board, just above the regulator, that says '1114-04M12'.

I'm using the LK204-25-V (wide voltage version) that is experiencing problems with certain keys on the keypad. Keys with the scancode of 0x4b thru 0x4f cause the display to behave erratically if the keypad is affixed to a non-grounded piece of metal. If the metal backing for the keypad is not ...