Toro ZS4200 Stopping

I have a ZS4200 that has all of a sudden developed a problem. It has been running great for 6 months but now it fails to keep running. The engine starts okay and runs smoothly but as soon as i move the drive levers from park to neutral the engine cuts out. I have checked the seat switch and this is okay. The mower will idle for every but as soon as one of the drive levers moves and the brakes releases the engine stops. Any ideas? Thanks.

Make sure the parking brake is off.