How good is your alignment, do you have a set up fixture? It won't tke more than a degree or two to make it drift at speed, especially if it is only one wheel that is off. If it is just one wheel, you can adjust trim all day and never find the sweet spot. How tight is the steering, any slop is going to have an effect. What are you using for a steering servo, stronger is better. Check your wheel bearings for wobble, that ain't good either. Hope this helps you narrow it down.