model 99E not keeping zero

just started having this issue. I can zero in at 100 yards,and get 1/2 inch groups all day. but when I go back out the next day it will be 6" low and 2" to the right. here is what I have done so far. checked my reloads( used same head stamped cases- trimmed all to the same length- used same lot of powder- same lot of primers and bullets- over all length is the same) checked mounts for being tight-changed scopes and rings- thought it might be forearm but it pulled forearm off and get the same thing- rear stock is tight. I am out of things to look for.last year I did not have this issue. it started this summer. is there any thing else I can try? this is a pre 1968 rifle. please help before I loss all my hair over this.
Thanks

When you changed scope and rings, did you check you base?
Is it safe to assume your using modern gunpowder?

yes checked the bases both are tight. using last years imr 4350.

How does the bore look? Crown? How many rounds through the barrel? How does the throat look?

every thing looks good.this happened all of a sudden I read to check action bolts which one are they talking about.
Thanks

Scub your bore.