JD317 bogs down after mower deck is engaged

I have a JD317 with the Kohler KT17 engine. When I engage the mower deck, I can cut grass for a few minutes then it seems like something is binding somewhere and the engine bogs down to almost stall. I have to shut off the PTO and it runs at high speed again. When I'm cutting flat or downhill it takes longer to slow all the way down but if I'm going up a slight grade or in thicker grass, it bogs right down and almost dies. I've checked all of the bearings between the electric PTO clutch and the deck. All are good. The pulley on the pto clutch spins freely when not engaged. I would appreciate any suggestions anyone has....I'm out of ideas.