92 celica gt steering vibration problem

Im having a wierd problem with strong steering vibrations above 60. They usually happen, but sometimes it comes and goes. The wheels are balanced and aligned, the cv joints are new. My mechanic is baffled, as everything seems really tight. The vibration when it happens feels very similar to the result of unbalanced tires, but they definatly are balanced. Any ideas would be a great help. Thanks
Derek

how about your rack and pinion, or else your steering column...ummm how about..ummm the boot by your axle if its ripped it will shake...i dunno just a few possibilitys

The boot on the axel is brand new as is the whole axel and cv joint. It shouldnt bee the steering rack, as the steering is nice and tight. Even the smallest movement of the wheels causes the steering wheel to move, so there is no freeplay. Thanks for the response though
Derek

rotate your tires. Also if your struts are bad you might have cupping on your tires