It sounds very much like the blocked exhaust syndrome, similar to what you get with a blocked spark arrestor.
So try removing the muffler entirely and see what happens.
THIS WILL BE VERY LOUD
compulsory ear & cranky neighbour warning.
IF it runs fine then there is a blockage in the muffler.
Also have a good look at the condition of the exhaust port, they will clog up if you have been using too much oil in the mix or been labouring the engine.
Check the condition of the bore & ring .
Husky use a single ring that works it's butt off running at 9,000 rpm.