Need JD 420 Garden tractor help:

Has Onan 218 (instead of Onan 220) and was running great for several years until tractor just stopped one day a few weeks ago after 30 minutes cutting grass. Would only restart after sitting for a while. When restarted the tractor was running somewhat roughly and the RPMs were very low. Changed oil and filter, replaced spark plugs and wires, new air and fuel filters. Still runs as before with RPMs not near where they should be and are very low. And as before, I can run the tractor for a while, shut it down, but it will not start back up until it has cooled again. What approach should I take to troubleshooting? Should I be looking for vacuum leaks, carb problems or governor/throttle adjustments? I have a 420 service manual as well as the Onan engine manual, but I am looking for advice on the best area to start investigating. I've already had to have someone else cut grass for me once and I've got some dirt to move!

...and thanks Dave T. for pointing me in the right place to post this!!