After reading many posts on cruise problems and supposed fixes, here is my story,

Having owned my E34 for 18 months and the cruise never working properly i hae replaced the actuator module (with a seccond hand unit) with instant satisfying success.

The cruise was always a bit lazy. would be slow to engage, surge and hold at NQR speeds and when the car was running warm or on a hot day it would not engage at all.

I removed the unit and "serviced" (cleaned and sprayed with CRC) for a small improvement in general control but it would never work when it was warm.

There are 2 seperate electronics in the module. a solenoid to engage and a motor to pull the cable. i can post pics of the insides if anyone cares.
it looks like one of them was just a bit tired and didnt have the balls to do the job when there was ambient heat affecting their environment.

The part was (wait for the laughter from the US readers) $195 "with 3 month warranty". yes that is seccond hand. the quote was 500 for a new module


At least it is now working and i can stop worrying about speed cameras on the freeways.