Before shutting down, make sure your mag switch actually turns off.
On the next cold starts, hand prop it thru all the four cylinders to
see if there is a dead one.

A stuck valve won't necessarily show as low EGT. I'd actually expect
maybe a high EGT but if you say the problem is short lived (for now at
least), you might not be able to spot it with the EGT gage.

How many cylinders receive prime? Has it always done this in cold
weather?