The slips just let the car think it is longer wheelbase than what it is, better weight transfer. If it would have been moving right off the launch, I would suspect the wheelie bar of steering the car, the fact that it happens past the 60' mark usually means the rearend is driving the car that way. One other thing you can do is swap sides with your slicks, if runout isn't equal that will drive the car also. If you find that is the problem, run the shorter slick 1 psi. higher. Or, pump it up to 40 psi and set it in the sun for an afternoon, that will make it grow some.