Sounds like a fault or interference to me.
Can you define "not far" as I have used am sets for years and generally they are perfectly adequate.
Fm sets are less likely to glitch but your am set should work well past 60-70metres without problem.
Usual causes of Glitching are faulty aerial or crystals, receiver or transmitter faults, interference from other sets or surroundings, faulty motors or motors with no capacitors, loose wires, speed control .......the list goes on. Make sure your aerial wire is clear of all power wires. I mount my reciver on the steering servo on my Tao5. ESC is behind the motor.
if that doesnt fix it you may need to one by one replace each component until you find the cause of the problem. If you can, borrow a mates receiver and stick that in and try it etc.
If its new, take it back and get the guys that sold it to you to help.
Your AM set should be sufficient until your skill level is at the stage where you want the adjustability of the dearer FM sets.