Tecumseh OV195EA won't run when hot

This engine is mounted on a ride-on mower that I suspect was built in Italy and which I've had for 4yrs from new. When the engine gets hot it starts to falter and won't accept any load (blades or wheels) it then stutters and stops, it will very occasionally backfire. When it stops and I crank it over there is still a spark. Once it has cooled it starts well enough and the cycle repeats. I've stripped and cleaned the carb, although it was free from gunge (carb seems to be a Tecumseh Craftsman - with no primer bulb or adjustable jets) and I've also cleaned the supply line/fuel filter, the oil is changed regularly. I've tried different plugs (currently NGK BP7ES) and I've tried it with the fuel cap removed. I'm waiting for a new air filter as the current one is a couple of seasons old. It's driving me mad and now after two days of rain the d**n lawn needs me!