You don't say which model and what length of bar. As it is getting progressively worse, I would remove the muffler and check for piston scoring and excessive ring wear. It could be damaged from overheating or just worn out.

Maybe it's adjusted too rich on the low adjuster, try turning the low adjuster clockwise about 1/4 turn and see if that helps. You can put it back if it doesn't. Steve

the description you are giving tells me the ignition module/coil is breaking down when hot.very common problem it will start missing then just die.wait about thirtymins to an hour and it will start and do the same thing. you might try some of the local repair shops for a used one. an index card should set you air gap