Hi Heather, long time no see, hope that you and Clive are both well?
As already said a Chrono string would help you eliminate internal problems causing the POI shift. If you have access to another cylinder perhaps you could try that as agian, it's an easy way to potentially rule out a problem. I think a bit more history is needed before we can give a solution.
Have you recently done anything to the rifle? has the problem suddenly appeared with no work done on the rifle?
I HAVE experienced the same problem as you a few years ago and despite me thinking that it couldn't possibly be the reg...it was
. If you do the chrono string and all seems ok and you need to borrow a reg checker then let me know as i have one and also a spare reg that has just been serviced, you could chuck it on and see if it sorts out the problem.