OK GUY'S, I need some help!!!!

I have a OS 46 FX that is mounted on a Model Tech Magic Extra 300 (great flying plane, poorly built plane, but that is another thread). I put this plane together in the fall, and the following is the things I think I did right and things I did wrong.

What I think is right:

1 - Break in engine with stock muffler exactly like the book.
2 - Use Cool Power 15/16 and boost the oil to 18% with an additive.
3 - This may be good or bad depending on who you talk to
I have a tendency to run all my engines rich!!

What I might have done wrong:

1 - Put a Mac muffler on the plane right after break-in.
2 - I could not get enough back pressure off the Mac muffler, and run the engine pretty lean. Quit a couple of times in flight. I took the Mac tuned pipe off and put stock muffler back on.

My problem is I have been fighting to try to tune the engine ever since. Each week I have to retune the engine, and for example I flew today the first four flight run great, then suddenly on the fifth flight it was too lean. Landed and richened it one click everything was fine. It seems to air temperature and humidity might have a effect, but I have a fellow member with the same OS 46FX with a Mac tuned pipe on a Somethin Extra and never touches the needle valve and it screams.

I also have the following OS engines on other planes and I have no problems with these, OS91FX, OS61FX, and two OS 91 four strokes.

Any opinions would be greatly appreciated!!

Thanks,

Jimmy