Hum... gonna take a guess at this one, but here goes.
Now, I suspect your car is running a four cell receiver pack (i.e. 4xAA battery in the car). First thing to do in that case is pull it and put a proper NiCad Rx pack or LiFE pack in instead. More voltage, and can deliver better current to the receiver and servos.
What could well be happening is that your throttle end-point isn't set, so at full throttle you're stalling out the servo and drawing a big load, which is either tripping the fail safe on the Rx (if it has one?) or simple pulling the Rx pack down to the point where it can't power the steering servo as well as the stalled throttle servo.
If it was me, I'd change the Rx pack to something beefier (assuming it isn't already), and then set the end point on the radio correctly (assuming the radio can do that). Also, if the Rx has a failsafe built in to it, as a lot of 2.4 sets do, make sure it's set properly... i.e. applying brakes!
Hope that helps some!
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