I noticed this at the end of about an hour of disking the other day. Today, I was bushhogging and after about 15 minutes the RPM just began to quickly drop. Engaging the clutch brought the RPM back to normal but engaging the PTO or transmission caused another drop in RPM. Yes, I'm talking about a significant and sudden drop. I limped back to the shed and shut down. Any ideas as to where to start in trouble shooting this? Thanks.