I have had the same thing happen to me twice now. I set for around 1800 and it ends up at 2000. The next time I gas it back up I'm going to check if the pressure reads 2000, or if it reads 1800 and then creeps up. Hopefully, it won't start at 2000 and creep to 2200.
I think your velocity swings have to do with the plenum size. It has to be something like that. You should try shooting a magazine as fast as you possibly can, and then try shooting another at an exact interval, like 30 seconds, and see what the spreads are, and do it with a full fill, and a low fill. That's what I would try.