The way you described the problem, I would suspect an air leak on a crank shaft seal where it seals onto the crank shaft (inner part of the seal that has a spring in it). With a minor leak there, it may only show up after running for a period of time. You could tune that out by opening up the low speed jet a little, but that would be just a band aid effect. But that would also tell you that the saw is getting too much air from somewhere. That leak is usually only detected with a vacuum test if its a minor leak. If you test only with pressure, often that part of the seal will not leak. It should show with a test on a cold engine. i would think the dealer would have done both pressure and vacuum.
