This is probably caused by radio interference. See Section 24.5, “Glitching”.
For brushed motors: the commmutator may be dirty. See section on maintenance.
Pinion meshing may be too tight. The pinion should not be firmly fitted against the main gear. The motor pinion should be able to wiggle just a tiny bit without turning the main gear.
Motor pinion may be slipping on motor shaft. If your motor pinion uses a setscrew, then your motor should have a flat spot on the motor shaft so the setscrew will not slip on the motor shaft.
If the motor shaft has no flat spot for the setscrew to bite, then you should make one with a Dremel and a diamond cylinder point bit (#7123).
For the Piccolo using the stock motor (G280 or G310)and the plastic motor pinions, it may be necessary to glue the plastic pinion onto the motor shaft to prevent it from slipping.
Woodies may be trashed:
Most larger wooden blades are made of a hardwood leading edge and a balsa trailing edge all shrinkwrapped together.
On hard crashes, the balsa part can be severely damaged but the blade looks fine due to the shrinkwrap holding the blade together.
If you look closely you may be able to see the breaks in the balsa. If you can see this, then the blade should be replaced.
This is normal. The helicopter will tilt slightly left or right because the thrust of the tail rotor is not in the same plane as the rotor head. Helicopter which have an elevated tail so the tail rotor is in the same plane as the rotor head (such as the Maxi-Joker) do not tilt while hovering.
Your headspeed may be too low and you may be compensating for this by using more main motor pitch which makes the heli motor bog down and the heli unstable. If the helicopter is fixed pitch, then you may need to reduce the main blade pitch so the hover headspeed is higher. If the helicopter is collective pitch, then you may need to modify both the pitch curve and the throttle curve to reduce the pitch and increase the throttle when hovering.
Your rotor head may not be firmly attached to the main rotor shaft. This usually occurs when the hole in the rotor head is reamed out after multiple hard crashes. If the rotor head is not fitting snugly on the main rotor shaft, it should be replaced. This problem is seen on MS Hornets. because the E044 pivot support is fragile.
On a CP Piccolo, this is usually caused by a broken pitch arm base (68211). Visually inspect the two thin "legs" which are between the arms and the tube. One of the legs may have broken.
The feathering shaft may be bent. Disassemble the head and put the feathering shaft in a drill and spin it to check if the shaft is bent.