This same thing happened to me. My dad and I replaced the power packs, and it worked for a while and then it went back to doing the same thing. Then, our mechanic hired a guy to hook up a computer to the car to test all the parts, and he found the problem. I am sorry I can not remember the problem with my car, but take it to a place where they will hook it up to a computer. I am sure they will find the problem.
Hope this helps,
Nikk