UK Car Repair: british toyota celica 1991, toyota celica, steering rack

Question
when i reach speeds of over 60 mph the steering is light and starts to pull to the left and right

Answer
It sounds like it is trying to get airborne! Does the front seem to be lifting? Is the back of the car heavily loaded?  Are the tyres the correct size and pressure? Otherwise it must be a problem with the steering rack. With power assisted steering, the assist should reduce at higher speeds due to less friction between the road and tyres???  Check and let me know.