model 99E not keeping zero

[whomeno]

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

[D.ID]

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

[whomeno]

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

[GaCop]

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

[whomeno]

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

[Robinhood]

Scub your bore.