I think you are likely right, but it also could be a frozen belt tensioner arm that is binding or the spring came off. Most belt slippage is accompanied with the smell of burning belts, but if the tensioner is not getting tight, you could just be slipping a bit. I'd check the drive belt to make sure it is tight and have somebody sit in the seat and energize the PTO while watching the clutch (stay well back from the deck, of course). I've found PTO wires tend to break from vibration and have repaired mine several times. If the plug to the PTO is loose, or a wire not making good contact, it could be your problem. BTW: You don't have to have the engine running to energize the PTO, so you can do that with someone sitting in the seat and not be afraid to put your hand in and check the belt to see if you can make the clutch slip. As you surmised, it's probably a failing PTO, but there are several checks you can make to be sure.
