Since you only mention a singular directional problem, the nose dips, this might point to an overheating or current draw issue with the speed controller, motor, or flight battery.

A radio glitch would cause directional changes of a more random nature. It sounds to me like the power system could be cutting out at brief, sporadic intervals due to the engine or ESC overheating or the power system drawing more current from the battery than the battery can deliver at a given instant. These kinds of problems would cause the motor to lose power for a brief time and result in the nose dropping.

If these nose dips occur at various speeds, my theory doesn't hold much water. If, on the other hand, the nose drops primarily only when you're flying at or near full throttle, this tends to confirm my theory.

To correct the issue, you might:

A) cut extra cooling holes at the rear of the cowl
B) upgrade your battery to one with a higher "C" rating or peak current capacity
C) upgrade your speed controller to one with a higher peak amp rating
D) try a different electric motor or two and see what kind of results you get

Good luck!