I have a bunch of Stil stuff...my weed eater is over 23 years old! Used it yeserday for over an hour. I do buy the Stil premixed, no alcohol, 93 octane gas at the dealer. Expensive but I've had no issues across the fleet since.
Check the primer bulb. They do deteriorate and can cause air leaks and "bog" at high revs.
Easy fix. I have a bag of them
Edit: FastFred is right. I have replaced carbs as well if the simple fixes don't work. Not worth the bug hunt. I just replaced one on a 25 year old mantis rotor-tiller: $45 online.