Sounds like you might have a bedding issue....I had bedding issues with my HS.
As you no doubt already know Remington actions are not round and despite what B&C or HS says it can play hell with accuracy in this type of a system.
Talk to guntech about the stock...He fixed mine (bedded it) then I sold it.

The guy I sold the stock to had to re-bed again.....
Is the mag box floating clear of the action (not squished in between the floor plate and action)?
Make sure the front guard screw is fully floating as well.
It can't touch anything but the bottom metal and the action threads.
Make sure the scope/mounts are right before you spend any money....I would never send a rifle out for work that had a scope/mount issue....RIIIIGHT!
Did you get the 8mag rolling?
EDIT
Just noticed you are using RL-25.
RL-25 drops velocity fairly quickly in cold weather and can vary significantly from lot to lot.....If you developed the load in the warm weather the load may have dropped out of the sweet spot.
RL-22 and H-1000 might be worth a look.
Ignore my velocity recommendations with the Accubonds.