I think your furnace is frying things on you to save your life my friend. I had a 25 year old suburban forced air furnace in my long long trailer and it started getting flaky like this. I diagnosed things, looked into prices of gas valves, boards, igniters, all the usual stuff.
I decided propane
is an unforgiving s.o.b. if not respected properly and I had more than just me asleep in there at night, so I ordered a brand spankin' new Suburban in the price ranges you mention, from PPL motor homes in Texas. Here is their link.
UPS damaged the first one in transport and PPL had another one on my doorstep in short order.
I put it in. Of course, loved it.
But then, for the grins and giggles of it... I autopsied the 25 year old furnace. Oh Boy, was I glad I did not spit and tape that thing together with new board or valve or whatever. Killing machine is all I can use to describe the insides of a furnace that old.
Remember, these things work by way of metal heat exchangers that need to keep the combustion on one side of the metal and the moving air on the other. Rust, cracks, holes, metal fatigue, dried gaskets... you get the idea.
Rust never sleeps my friends, but I will, and not with a furnace that old believe me.
Plus, I promise you that whatever you replace today, the next part will fail right when you don't want it to. They last a long time, but not forever.
Just my two cents. Keep the change.