It could be that you have a weak steering servo, its not a bad idea to upgrade the stock one anyway. I have to say though, it sounds like you are describing the effects of centrifugal force. The faster your car goes, the more it just wants to go in a straight line. There is nothing wrong with your car, its just the effects become more apparrent at higher speeds and on a loose surface. A stronger servo will fight understeer a little better though so thats where I would start.