car: 95 850 2.5 20 valve auto, milage 100k

Problem : braking from 70 down quite a 'sharp' vibration and noise. Not detectable under 40 or so.

History : 18 months ago circa 30k ago both front calipers and discs and pads were replaced to cure sticking calipers - genuine volvo parts i believe.

3 thousand miles ago the front calipers were stripped and cleaned, disks and pads inspected (8mm left on all pads, disks well within tolarance) and everything put back together - note torque wrench used for all assembly. (by me)

1700 miles ago one rear caliper was replaced (sticking) along with both disks (brembo) and all pads and handbrake shoes. (by me)

Rear tyres are 40k old and half worn, one front tyre is 5k old the other was replaced 1000 miles ago (tyre fitter witnessed to use torque wrench to re-apply wheel!).

The problem started this morning, and is - i think - getting very gradually worse I have done 150 miles today)

On the way home tonight i stopped and tightened all the wheel nuts using an 18" extending wheel wrench, although i put 1/4-1/2 a turn on all the nuts nothing felt loose or of a particualrly difernt torque (nor overtight for that matter). This didnt effect the problem at all,

There is not really noticable pulse in the pedal or shake in the steering wheel.
All wheel have been felt by hand an none seem hotter than their opposing number on the other side of the car (fronts warmert than rears naturaly).

Now i have never had warped rear disk and thats ny first thought (due to it happening durring braking and the nature of the vibration - it really feels quite sharp, i would expect a warpesd front to be damped a bit by the sliding pin arrangement of the caliper)- has anyone else and did it feel as I am describing?

My betting is a warped disk (possiby a warped alloy though i dont know why...) or possibly a wheel out of balance (though it feels to much for that it really gets your attention!).

The plan is to do the following on each wheel in turn:-

Jack up the car
Use a dial guage to check run out on the ront surface of the alloy
remove alloy
Use dial guage to check run out on disk

If I find run out on a disk i intend to remove, clean all maiting surfaces and reseat and check again...

any thoughts, advice or sugegstions for other causes/checks i should do greatfully recieved..