Mystery ailment

I don't know how a 2-stroke engine can be so mysterious. My Stihl FS76 trimmer starts fine, but then runs erratically, speeding up and slowing down according to its own whims; then dies. I have replaced the plug, carb, fuel line, & fuel filter. Have tried it without the air filter in place. Have tried adjusting the carb. I can't think of anything else to try! Any ideas?

How many pages do you want to be typed ?
Apart from the carb, the coil can be going bad, the kill switch or the wiring could be shorting.
Then there are 2 seals that need to be airtight in both directions plus an impulse line that needs to be tight so the pump can work.
The tank needs to be pressure tight and the tank vent has to be working properly.

Suggest you go to U-Tube and check Joe Pace - two stroke diagnosis.
Just because you can buy one at any hardwear store does not mean that the technology has changed and mechanics spend a year learning how they work & how to fix them.