I had a similar issue a couple of years ago. The steering would switch between light and heavy, not good especially going round corners.
Changed the valve, no difference.
Spoke to main dealer, 'sounds like you need a new steering rack'
The problem was the knuckle between the column and the rack. It just needed a good clean out and re-grease.
Job done, never been a problem since.
Got to be worth a look.

New steering rack my a@@e.

I have a similar problem - steering is really heavy at low speed - my indie reckons its either the servotronic valve, the electrics or the knuckle itself - seems like you're looking in all the right places. I've had a good look at the knuckle on mine & can see rust around the mating faces - I've drowned it in WD40 & will give it a few more soakings over the next few days & report back - if it makes a difference, I'll try to strip it down & clean completely & re-grease 2003 L322 Vogue SE 4.4 + LPG
2005 Aprilia RSV Mille
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In the face.
With a chair.

interesting, let us know how you get on [with pics if you can].. ... - .- -.




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Update: had servotronic valve renewed by trustworthy indie (genuine brand new RR part) - no difference, steering still heavy at low speed. Changed BCM with one from a 'known good' donor vehicle - still no change...
I'm back to thinking it might be the knuckle between column/rack - the first indie i spoke to was sure it wasn't that (although he was also the one that was sure it was the servotronic valve...) My only concern there is that if it was the knuckle, I really don't know if it would create enough resistance for the power steering to be unable to overcome it - any thoughts anyone?
Taking it to the local Stealer on Monday, to see if they can find anything electrical 2003 L322 Vogue SE 4.4 + LPG
2005 Aprilia RSV Mille
__________________
Some people just need a high-five.
In the face.
With a chair.

It could be "upstream" of the steering housing and then the power steering does not make a difference. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

Did we ever get to the bottom of this??