codykrr: make sure that the spring on the throttle linkage gets a little pressure at idle position. This will make sure that the carb wont open up because of vibrations from engine. or perhaps your carb settings are too lean?

I used to run the car with alkaline batteries before, and a good sign that they needed replacing were servos going crazy. notice that when you turn on the power on the car, the servos will move a bit before they stop at their correct position. dead alkalines make the car turn on and off, on and off... dead nicads make the servos slow and weak.
I would recommend a 6v hump pack for your cars. dont put rechargeable batteries in the original battery holder, then you will get 4,8volts instead of 6..