The newer o-rings would have NO wear on them, meaning they would be ever so slightly larger. Once you had used the old ones for a bit, they lose a few layers (While looking perfectly fine to the human eye) and would have a looser "grab" on the inside of the bolt.
Mine personally wont fall off either, I have to pull my bolt of the guide, but mine shoots 290+-4 on a 79 insert.
Doesn't seem like anything is wrong here, other than that your reg may not be fully broken in yet and is occasionally giving a spike of pressure.
Dust Black Axe w/ white accent
14" Freak Kit
Spire with CrownSF
Crossfire 68/45 w/ Stealth