Toro 190cc B&S "Ready Start" engine mower

I have a 2yr old Toro self-propelled walk behind mower. It starts on the first pull every time. BUT, lately after the engine has been running for awhile and I shut it off I have to wait 10 to 15 minutes before it will start again. I replaced the spark plug, cleaned the air filter and checked the operation of the choke and thermostat hoping one of them would resolve the problem. It didn't.

Any ideas or help would be GREATLY appreciated.

Thanks!
Lee