What size prop are you running? Which glow plug? Which fuel? Are you sure that the pressure fitting on the muffler is clear and open?

Some fellows flying models with radial engines (scale) over compensate by running a larger than normal prop. This lowers the rpm and the cooling air over a new engine (you do have an exit area for the heated air - right?). This extra heat also advances the timing, which will make your mixture too lean, especially on a new engine. Three tanks isn't really enough to break-in an even ABC engine properly.

If you are running a larger than normal prop, but need the diameter to get out past the cowl on the AT-6, consider dropping down in pitch an inch or so, unless you are already down to 6" of pitch. I wouldn't go lower than that for a scale type of model. They are not 3D floaters.

Also, make sure there is plenty of exit area for the cooling air once it hits the hot engine and expands.

Frankly, a few more tanks of fuel (up off the ground so you don't wear out the engine by sucking dirt through it) just might make your problems go away.

Get back to us and let us know what's up. Good luck.