I have a 42" Craftsman riding mower with a 18.5 HP B&S engine. It started using oil suddenly a couple of weeks ago. The compression is 120#. What could be the problem?

Also after it has been running for about 10 minutes, it shuts down. It does this with fresh fuel, new plug, clean gas tank, new fuel and air filters. Sometimes it restarts and at other times it won't. What could be causing this problem?

The engine is, model-31P777, Type-0299E1, Code-04805ZD.

Thanks, George