Your unit may be surging. Did you try cleaning the jets in your carburetor? There is a thread on this site for surging issues in relation to the Lawn Boy Dura Force 6.5 HP engine. Specifically the thread is on the 10550 edition which is the one I have. I drilled the jets to enlarge them slightly as recommended. But in the process of cleaning and fixing the carb, my governor spring got overstretched and completely disengaged from the vane. But with no hardware visible I can't seem to get the vane off to replace or even service the spring!

Thanks for the help!