John Deere Riding Mower Stalling When Clutch Released?

My John Deere riding mower, LT133, stalls every time I take my foot off the clutch. I can start the mower and it will run fine for a minute or two, but then it starts to stall every time I let the clutch out. It doesn't matter if the blades are engaged or not. It also doesn't matter if it's in gear or not. As long as the clutch is pushed in the mower runs fine, but as I take my foot off the clutch the motor stalls. If I push the clutch before the mower cuts off the mower continues to run fine, but then when I take my foot off the motor cuts off. Can anyone suggest what could be wrong?