Crap tyres?
My first Cougar did the same til I ponied up and bought more expensive ones - it was also 'tram-lining' badly whenever there was an uneven road surface (incredibly rare in the UK, I know...lol)
Look also at the age of the tyres - the ones on the Cat I just bought were manufactured in '03 - meaning they're well past the manufacturer's recommended use-by date (5 years max with year-round use, regardless of tread-depth).
Here in Tchermany if I try to TuV it (same as the MoT but more stringent), that'll be a fail or an advisory depending on the age of the tyre. In this case - 10 years - it'll be a fail.
Also if you're using Summer or all-season tyres and the temperature is below 10C it'll affect the performance even when new and even more so if they're old as the tread blocks lose the ability to conform to the road surface as they were designed to do as the tyre's elastic properties gradually fail with age.
Other factors can change the tyre characteristics: when I parked-up the car for six months back in '06 when I was deployed to Iraq, the car was sat still throughout the winter and when I drove it again those tyres were no longer any good, even though they were within age and tread limits, having been put on 12 months earlier - it was like driving on solid wheels.
When I changed the tyres over to new ones, it was like having suspension again after driving on solid shocks and the car no longer felt 'skittery' when cornering.
and that was cheaper to tax than her 2.0l 54 plate focus lol
