Actually it makes a difference.

The lower pitch puts less load on the engine, which is what we are trying to accomplish!

A lower pitch will drop the load on the engine, but a smaller diameter drops it even faster....

So a 12x6 versus a 13x6 will present less load on the engine than a 13x4 versus a 13x6 prop.

As I previously said 13x6 is IMHO too large for that engine. Even my .75's will bog down with that size prop, which is just about right for a .75 and definitely not for a .61 let alone a .55AX as you have. All of your symptoms are consistent with too much load on the engine.

As Jimmy says if you want 3D capabilities try the 12x4. That will produce more thrust but less airspeed.

BTW: Get yourself a fish scale. Knowing the weight of a plane is a useful measure especially when evaluating or correcting behaviour.

A cheap fish scale will do fine, nothing fancy is needed.... you can also use them to measure thrust too!