Cylinder head cracking !
If you suspect plug fowling then running at max RPM is OK but don't lean off as the rapid temp rise is not good for the cylinder heads.
You might also like to try the Lycoming recomended shut down- run the engine at 1600-1800 RPM for 30-45 sec then set 1200 RPM and shut the mixture.
This is stated in a Lycoming SI to cut down on plug fowling.