A couple years ago I bought a .308AR (a Rock River LAR-8), put a Vortex 6-24 PST on it, mounted with a 20moa weaver mount and a Vortex one-piece mount/rings.

The rifle shoots great and is very precise, but the zero shifts anywhere from 1.5 to 8 MOA every time I take it out. It shifts both vertically and horizontally. It does this at 100-yards at an indoor rifle range, so environmental conditions are pretty consistent. I've been using the same batch of factory 168gr Hornady A-Max the entire time.

I want to fix this; I suspect that the cheap mount and rings may be the culprit. Before I spend money on rings, is there anything else I should try?

I don't "bang the rifle around" aside from it riding to the range in my truck's back seat. I don't needlessly mess with it. Also, I used a torque wrench to properly torque the mounting screws and marked them with a dab of paint to make sure they aren't backing out. I have an SR-15 with an ACOG and it does mot have this problem; it has stayed perfectly zeroed since I got it, so I don't think it's me that's causing the problems.

Any ideas would be greatly appreciated

Disassemble the scope and mount and put it all back together. Use loctite on the rings but not on the base
 

I assume your base is not bridging the receiver and the rail?

When in doubt like that I always check the mechanical things like mount screw torque/looseness.  

If all that looks ok, then I question the scope and substitute a spare and do some test groups.  If the spare works fine, it was something with the original scope and send it off for the maker to check out.

Joe and Steel are both on it, the only other point i'd check would be torque on the mounting hardware
a hex-torque wrench is pretty standard fare for this kinda work

To add to the other good suggestions, rest the gun well, look through the scope and crank the elevation knob up and down, you should see it move consistently with each click and change direction when you do. Repeat for windage.

Could it be the scope?  My father is having this very issue.  He is shooting one of the DPMS .308's, scope is the 4-16 Viper HST.  Both gun and scope are 2-3 years old now.  He zeros the rifle gets it shooting good takes it home and the next time he shoots its still grouping good but its zero is off.

I am not sure if he has tried the scope on any other guns or different scopes on this gun but I know when we mounted the base and scope everything had blue locktite and was properly torqued.

This is happening with a Strike Eagle my buddy has too. It's ridiculous.

How are you resting the rifle?

I'd check the scope.

8 MOA is too much.
I wanted to ask you how do you carry your rifle. Carefully in a hard rifle case or a soft one? Is it in a good place in your car?
The prevoius suggestions are very very good. But 8 MOA is too much.
There should be something loose. Other question I would like to make you, is if your zeroing keeps zeroed during the hole dhooting sesion or if it moves.

Retorque barrel nut with ft/lb TQ wrench. Retorque scope mount, with in/lb TQ wrench. Retorque scope rings, with in/lb TQ wrench.

Clean barrel before first shooting session to remove copper.

Do NOT clean barrel for 3 shooting sessions.

Use solid sand bagged rest for groups that allows gun to move straight back, don't push cheek into gun, grip hard or do anything other than just contact the stock.

Shoot only 1 round per minute, 5 round groups.

Report back.

Pics? Side view.

Rico,

any change after suggestions?
or did you find out you were adjusting the knobs during regular handling?

I'm just wondering, it's been a while and you kinda fell off the map

I had the Same issue a while back.
I had a 2.5x10 Weaver Tactical FFP mounted in a Burris Pepr QD Mount on an AR15 and it was Rock solid and never had any POI shift. When I whapped it to an AR10 it was all over the place. It turned out the Burris Pepr QD Mount was shifting on recoil. I Sold that POS local and got set of Weaver Tactical rings that were loctited  and torqued to spec and its now Dead on with Zero POI shift.

My buddy was having problems with his scope, turned out that the bell was pressed against the barrel.

Seems it should be obvious, but he didn't notice.

I put it in a rest to verify, messed with the adjustments and noticed they weren't acting right, then noticed the interference.

It was pressed to the barrel hard enough that it bent the scope body so he tossed it and got one with a smaller objective.

Check all mounting and make sure the adjustments aren't moving when you put it in/out of the case.