Gas Weed Eater only runs for 20 minutes a day

I have a 4-cycle Ryobi weed eater that, starting this season, will only run for 20-30 minutes. It starts great with 1-2 pulls and runs perfectly at any throttle, but it gradually stops working at higher throttles. What I mean is that it will run fine at 3/4 throttle but going beyond that kills the engine, over time that max goes to half and then 1/4 and eventually it will only idle and then dies completely. Letting it sit for an hour does not fix the problem, but leaving it overnight will restart the cycle of getting 20-30 of use out of it.

The unit is about 6 years old, but I had a lawn service most of that time, so it has only gotten about 1 or 2 seasons worth of use. I have done regular oil changes but no other maintenance.