Ran the 18x10 some more today and 4-5 minutes into the my second flight I lost the top end.   Landed and the motor had tightened up pretty bad and it didn't sound very good turning it over.   Went back to the 18x6 after a long cooling period and it was certainly much happier with that prop.  I have the 15.5 on order.   What the heck happened... I am thinking I got it way too hot.   It wasn't air flow.