I just returned from a trip from San Diego to Peoria for Spring Training, in my 04 XJ8 with 57,000 miles.

Although my car performed flawlessly I was a bit disappointed with it's high speed stability when compared to my daughters S-type I had driven from CA to Ohio last fall.

What I noticed at speeds over 90mph was the XJ seems to nose dive a bit and bounce or almost float more easily with rougher road conditions (humps or valleys) and the car simply seems less stable at high speeds than the S-type.

I have had the compressor changed, the front control arms, it does have older tires that are about 50% and I get a little steering wheel shake from 60-70 which I think is a tire balancing.

In contrast the S-type feels heavier less floatie (can't think of another word) and holds a line better and is quieter inside the cabin.

Do you think it's the tires? Maybe the fact that I had a head wind? Any other ideas? The car has all four original shocks but they all seem fine.

 

According to one of the technical description documents the dampers in the air suspension units ('air shocks') should switch to their 'hard' setting at speeds above 90 mph.

Are you sure you were going above 90 mph ?