This is interesting.... I gave up shooting 77/97s after experiencing the same.
Zero at 30, all good for days on end then half way round a comp I'd be almost flat at 45?!
Happened a few times and never did get to the bottom of it. It was also a VGLIDE but I don't think thats it, I was sure it was stock/bolt related. I gave up and got a TX. TX: Solid non-movable forend bracket, 77/99 dodgy movable bracket. huuuuum.
tried over the chono too and never had any problems show up. drove me crazy. It would even settle back down by the end of the comp grrrrrrrrrr.