Tanaka string trimmer starts and runs for 5 minutes

Tanaka PBC-215 string trimmer runs for 5 minutes then stops. Will restart when cooled down.

What has been done:
1. Clean fuel and new fuel tank filter, new fuel lines.
2. New fuel tank cap.
3. Remove and rebuild carb with Walbro carb kit, new gaskets all around.
4. New spark plug and air filter.
5. Cleaned and inspected start stop switch.

My suspicion is that the ignition components are heating and expanding. There is a coil and and transistor so it might be either component, thinking ignition coil cause it sits right on the engine. Was doing this last fall and should have put a spark tester on it to validate spark, but didn’t do that.

Coils for this unit are hard to find and near $60, the transistor component $50. If this were a replacement carb for $20 I would buy it, but can get new string trimmer for $250 with a 5 year warranty. This unit was purchased in early 90’s.

I have put a lot of time into this unit and hate to throw it out. If I knew it was the coil or transistor I would replace it. Prior to this issue the unit has run strong and is an excellent trimmer.