Initial oven set stubborn

Our 1994 Jenn Aire 27" wall oven is stubborn about implementing our initial bake settings. When we place food inside, dial in the temperature, and then hit the "Bake" or other touchpad, we hear the usual clicks from behind the control panel, and the readout says "140" degrees to start. That's all normal. But then the entire thing clicks again and goes off. We've discovered that repeated attempts followed by hitting the "Bake" or other desired touchpad again right after the initial clicks will get the panel to say "Pre-heat On", and that often gets it working. But at times multiple hits of the touchpad button while the panel's still showing a readout is the only way to keep it on. It's very aggravating. If the readout holds for 10 seconds or so without turning off, it's usually good to go, and in a few minutes temps begin rising as designed.

I can't find anything wrong with the large control PC board. But it's vent slots are right at the top, which is below a microwave built-in above. Occasionally water and stuff has dripped down onto the vent area, so I added a diverter/shield inside the oven's top panel so anything entering via the vents were guided away from the board. So it's now protected, but has gotten pretty cantankerous about starting up. The wife had a devil of a time last night getting the oven to work.

Why does it take our settings, seem to start an On cycle, but then quit?