Stihl FS-36 String trimmer runs at low RPM and underpowered.

My string trimmer starts just fine, but it lacks power. That is it boggs down easily when comming on contact with actual grass. The engine rpm seems a bit slower than it ought to be. When at idle, it is prone to stalling.
This sindrome seems to have snuck up on me slowly rather than have come about all at once.
The air filter is clean. I changed the gas. I filled the spark plug and cleaned it up the tip with some sandpaper.
This has no effect. When the SP was pulled it did appear rather gunky but that is pertty subjective.
The machine is about 5 years old, but It probably has all of 25 hours on it.
Any thoughts would be appreciated.
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