1st time I've owned a car with factory front and rear 20" wheels and low profile tires with the Brembo brake pkg. Tread is good and sizes are correct OE recommended.
When driving highways with soft left and right curves at speed (70) the car begins to wag thru the curve, correcting itself when highway gets straight again. I've seen p/u trucks with trailers do this but never a car. It feels like the car is flexing on the tire walls. Road surface and ambient temps seem to have make no difference.

Any experience or suggessions?

 

Are they Dunlop Sport Maxx? I had those on my car and I think they where OE at the time for the XKR, they are crap tyres. Mine used to lose grip out of nowhere even in the dry, they where even worse when it got slightly damp. They weren't old or anything I just don't think the compound was very good. But Dunlop don't do all that well in tyre tests anyway, I upgraded them to Michalin Pilot Sport 4S and I like them a lot but I'm going to upgrade those to Michalin Pilot Sport 5 soon when I got to 18" wheels. I assume you're car has CATS suspension, unless it's been downgraded to the non-adaptive shocks. Easy to check by making sure there is a cable attached to the top of the shock absorber you can see when you open the bonnet and remove the black plastic cap where the shocks mount.

Could also be the tracking is out because the front mounts collapse dropping ride height and the rears do the same plus the springs sag with age.

 

I think that the problem is more likely to be worn dampers, or similar, than a problem with the tyres. Does the car have active suspension? Is there any clunking from the rear end as worn bushes in suspension/sway bar components will also cause imprecise tracking.

 

If its the dampers (shocks) then if you drive one wheel over a reasonable pot hole at say 40 mph it should be quite loud and send the car into a shudder.
Of course some or all of the rear suspension could be loose or corroded/broken from its mountings. Mine has 110Kmiles and the original rear shocks are still good.

 

Not familiar with active suspension but failed upper shock mounts will cause these symptoms on non-active suspension.

 

Two of my previous cars (not jags) have had this feeling, on the first it was worn strut tops (upper shock mounts) and the second it was loose bolts allowing the trailing arm assembly to slide from side to side. I believe this symptom can also show up with clapped out bushings.

You didn't mention tyre pressures in your original post so I'd eliminate them first, then check strut tops, then jack the car up and start looking for play in the suspension.

 

The open rear diff might cause that feeling but most likely is due to the alignment being out of spec.

 

As all our cars have open diffs you can count that one out

 

The open diff in the R creates that feeling when on throttle, especially in the wet, if you are used to LSD.

 

THX for the input. Tires show no obvious signs of alignment wear, but that might not mean its not out of spec. Local specialty shop charges $150 for 4-wheel OE spec alignment. 10X less than throwing on new shoes.

 

There are non so blind as those who cannot see