NOT replacing the existing resistors, they are AFTER the de-bounce capacitor.
Adding a resistor in the wire from the trim switches will drastically reduce any spike caused when switching. Not having a resistor is, in my opinion, poor design.
Flaps: Since you have a Tx with the problem, perhaps you are in a good position to try this fix. A 1K resistor should be fine, the capacitor will discharge inside 1mS, but the current will be limited to 5mA. The 1K will also provide a low enough input voltage to the processor to guarantee a logic 0.
If this does fix the problem, then it sounds like it should be done to all 8 trim switches, the 4 direction buttons and the MENU and EXIT buttons.
Maybe I can find what the effect is that causes the reset, e.g. is it the regulator, a processor brownout, the processor seeing a reset pulse, or what.
A thought, are the spikes, that are likely a negative spike on the ground signal being coupled into the processor reset by the large tantalum capacitor on the reset signal. Port D, the trim switches connections, is close to the RESET pin. May be a first test is to remove the RESET capacitor and see if that solves the problem.
I'll see if I can get my '128 board in a Tx for testing.
The difficult we do immediately,
The impossible takes a little longer!