MPS6 steering wont return to centre

Ive had my 2005 MPS6 for a year now, in the beginning the steering wouldn't return to the centre properly. I got an alignment and that fixed the issue, but it slowly returned over three weeks. Had a second alignment done, and it fixed the issue again, only to go back to usual about three weeks later. I then got it checked out by Pedders, bloke said the steering rack had gone and so I swapped it out for a refurbished one, steering has felt fresh and accurate since, no play, nice and tight, however now the steering wheel now doesn't return to centre even after the alignment trick.

The power steering also feels far too eager in the first 10 degrees of turn left or right in the sense that, if I turn it slightly, it wants to jerk the wheel a little bit more in the direction I turned, so I have to be wary and resist that tug. It drives fine, however lane changing is scary if i need to look over my shoulder cos I don't know where I'm headed if the wheel jerks over a little bump etc. It's not something I'd let a friend drive because its too unpredictable.

I thought maybe the caster was out of whack but have since learned that its a fixed, unadjustable setting without aftermarket stuff.

Anyone experienced a similar issue with a car before and have any advice?

Thanks in advance,
Luke

I've had steering wheel put back on crooked after replacement. I expect you could see same replacing the steering rack. So centre isn't center and super annoying.

re: sharp turn-in; I'm going to suggest that could be the toe alignment, possibly....but more than that I'd hunt down someone fairly local who is very familiar with the model. It shouldn't be darting around in unpredictable ways....

You might be noticing some sharpness because steering rack isn't worn out? Might be normal for good rack but not used to it?
re: centering - I'd try to figure out if the steering wheel is just aligned off center or if there's something else going on.