If you have allready checked all the basics such as pressures treads,side-walls. barings, wishbone bushes etc and tracking...then you might be feeling the same stuff as I did of late with thunderbird one:
I have had a symilar experience in my Black 10 ....
It was noticable only after I had my low profile tyres fitted all around
When I had Low son the front and tractor tyres on the back I didnt notice it...Only with 4 all around
In feer that ther was something in the steering or suspension comming loose I had it thoroughlt checked by two independant garages and both places gave the car the clean bill of kealth machanically.
One garage commented though that the drive felt 'twitchy as !"£$' (my conclusion here was that he was expecting a drive-feel the same as a stock K10)
Anyways I came to the decision that the problem was that I was sensitive to tramlining caused by the low profiles on the rear
Now, about 3 months have passed, I havent changed anything on the car but if feels solud as a rock now...so I reccon I'm now de-senstitised to the tramlining (my pea brain has filtered the sensation)....now I only notice it on road markings or visibly uneven road surfaces