On the inverted engine... its probably not fully broken in yet.

An engine can be fine upright, and a dog inverted when its still breaking in, and that can take longer than many people expect.

Some .40 class engines still aren't fully broken in after 3 to 4 gallons of operation. (That's a lot of flying...) Most will stabilize sufficiently for short inverted ops after 1/2 gallon... but for inverted mounting, I find it better to use an engine with a year or two of operation history.