Switch is being mechanically overstressed?
Now a valve problem which happens to have the same symptom?
To tip the 50-50 odds in this case you need to measure the voltage to the valve or the current through the switch, whichever is easier.
What a hassle. If this were a car there'd be a Technical Service Bulletin out on this problem.
"For any two correlated events, A and B, the following relationships are possible:
A causes B;
B causes A;
A and B are consequences of a common cause, but do not cause each other;
There is no connection between A and B; the correlation [which is implied by having the same symptom] is coincidental."