Hello gentlemen,
I experienced (and solved) similar haunting vibrations in my '95 LS400. At 55K a new set of Goodyears where put on the car, and as they broke in odd speed-sensitve vibrations would occur: front wheel hop, rear wheel hop, heal/toe oscillations, pulling in either direction,...you name it! A different problem would start as old ones faded away. It was not shock, spring, balance, or inflation problems.
I tried everything to solve the problems, and consulted countless "experts" in vain. Finally I realized the problem wasn't the suspension, it was the side-wall stiffness in the Goodyears. Temperature, air pressure, and road condition variables mask the problem, but replacing the tires with BFGoodrichs returned the car to its original ride!
Many tires that are claimed to be rated correctly for this car simply are NOT. This car demands a very stiff sidewall, so I'd look there for the solution.
Best of luck!
Dave
