It's hard to say what the problem could be. One way to check is to connect a servo to the throttle channel of your receiver, operate the system from a distance while you walk around the model to simulate its flying around. If the throttle servo acts up, then the problem is in the transmitter or receiver. If the servo operates correctly, we'd start to suspect the speed control.

One thing to consider about electric systems. If your speed control's not acting correctly, it could be pulling enough current out of the battery pack to cause the voltage to drop and the throttle failsafe to kick in.

Also...while you may think your battery packs are good, they may actually be marginal, so when asked to deliver the current the motor requires, they have a significant voltage drop that activates the throttle failsafe. You may wish to check this by using a brand-new, absolutely good battery pack that's been carefully charged and tested to be sure it's holding its full-rated charge. Older cells may not be able to deliver the current required without having an unacceptable voltage drop.

Please remember that ANY battery pack will experience a voltage drop when current is drawn from it. The more current, the more voltage drop. Good battery packs will have minimal drop, bad packs will have a large drop.