Couple of questions for the pair of you:

a/ what size wheels and tyres do you have?

b/ you say the tracking is fine, but what are the measurements, tow in, toe out?

c/ is the steering rack central, i.e. exactly the same number of turns lock to lock from centre?

Reasons are because larger wheels and tyres can cause light-handling
If you have too much toe in, the steering will be very light and will not want to self-centralise, causing instability
If the steering rack is not central, then you will not be driving in the "dead-spot" where the power assistance is @ its minimum. This is a small area on the rack spline (internally) where the PAS pump does not run under pressure. IF the steering is off-centre, then the PAS will be working continuously (normally more noticable one way than the other) and the steering will feel light and twitchy.

Check the above and come back to me with the results and I will see what I can sort out....