3500A816 Randomly wont start or shut down

[Smithcraft]

Hello!

So I have a Coleman 3500A816 (No A/C) and sometimes the blower won't spin up but the heating elements heat up, then it shuts the elements off to prevent them from overheating.

Then this morning, it wouldn't shut off and was continuing to heat.

I messed around with the thermostat (Honeywell programable) a bit to get it to shut down but it didn't, so I ended up flipping the circuit off.

Then when I got up, I flipped the circuit back on with the thermostat off, and one of the wires disconnected, and it still fired up, so I switched the circuit back off.

A bit later, and just for fun I flipped the circuit back on with thermostat off, and the furnace didn't start. I went to the thermostat and turned it on, and after a few seconds the furnace fired up. I turned the thermostat off and after a few seconds the furnace turned off.

It's now functioning the way it should.

So from reading this fine forum it looks like I might need a new sequencer (3115-3571). Suggestions for more component testing would be appreciated!

Since it's not available locally, it looks like I have to order it online. Are there any other bits I should order for a 30 year old furnace?

Thank you.