Throttle Lever won't stay on Fast, keeps slipping to Slow

I just bought a used Honda HRR216VXA (on Craigslist) and used it the first time today. Got it going fine but every 30 seconds or so, the throttle lever would slip back to the slow position. It was extremely annoying--I kept having to push it back up to Fast. Is there a possible quick fix for this or does it need to be replaced?

Throttle Lever won't stay on Fast, keeps slipping to Slow