Possibility#1: Your servos draw more power than your receiver pack can provide resulting in the brown-out effect you're getting.
#2: Bad receiver
#3 Bad power switch or some other short in your wiring.

Either way, you should check your fail-safe settings. At the very least, your throttle servo should be set to go to neutral or apply a bit of brake when/if your receiver ever looses signal.

If you do wind up upgrading, I'd take a look at the Futaba 3PL in your price range. Futaba's are known for their range and quality and it should fit your needs well. It also won't have any of the lag issues that plague some of the much-cheaper radios out there.