Mysterious Vibration-Can you Help?

[guycrosby]

I have a 1971 911T, which I have owned since new. It has about 64,000 miles, and has developed an occational mysterious vibration. The vibration appears only when the car reaches about 65-70 mph. It usually occurs only once during most times that I drive the car at this speed. After about 5 minutes of driving at this speed the vibration stops, and usually does not return until the next time it is driven (that is, on a different day). The vibration is not strong, and resembles something like driving over a fairly gentle washboard, or a tire that has lost a weight. It appears to be from the rear of the car. I recently put on new tires, but this did not solve the vibration. My mechanic (Jerry Pelligrino, European Performance Engineering) checked the suspension at the time of fitting new tires and could find no problem. The shocks are the original konis, which the mechanic said were not leaking and appeared fine. The car is driven only occationally, and can sit for weeks between drives (partly because I also have wonderful 1972 911S to drive).

Does anyone have any suggestions about what might be the cause of this mysterious vibration?

Guy

[Eric911S]

hello: I would check your cv bolts, to make sure they are tight. I would also check the tires for being round and balanced. If they are your original shocks they are going to be bad or at least not good. The shocks could be leaking internally and not to the outside. You can take the shocks off to see how they are. Lastly are you sure the vibration is not in the front? Good luck. Thanks Eric