My JD z225 w/ 18.5 briggs engine has been stalling after starting the last few weeks. Sometimes, when the PTO is engaged with the engine at full speed, it will also stall. This week, it was running fine, I had been mowing for about 1 hour when I shut it off to move something. When I went to start it again, the starter gear engages the flywheel gear, then stops dead. I tested the battery 12.55 volts. Any ideas?