John Deere LA130 engine dies when mower engaged?

I have a John Deere LA130 lawn tractor that starts fine, but when I pull the PTO switch to engage the mower, the engine dies. When this first happened I could engage the mower if I wasn't sitting on the seat (it shouldn't have remained running because of the seat safety switch, but it did), but when I sat down the engine died. Now, when I start the engine it dies as soon as I pull the PTO. Also, the engines remains running whether I'm on the seat or not. I haven't bypassed any of the interlock switches.