That was discussed at length on the Yugoslav Mauser forum which, is what that link goes to.

That issue is also discussed on the Forster site here--- http://www.forsterproducts.com/store.asp?pid=26986
They provide gages for either shoulder angle.

This may sound crazy but, have you thoroughly cleaned your bore and chamber?  I do not mean to sound condescending but, you would be surprised at how often this simple step solves the problem.

There is one other potential source of chamber fouling; lacquer.  This problem is more common in Mosins due to lacquer coated steel cased ammo being the norm for them. But, with proliferation of Romanian and Russian made lacquer coated steel cased ammo for the '8mm' this could be the problem here.  Shooting lacquer coated steel cased ammo softens the lacquer which can build up on the chamber walls.  However, lacquer is not affected by most bore cleaners. Only 'lacquer thinner' will cut it.  Try chucking a camber brush or shotgun brush to a cleaning rod segment.  Chuck that up in a drill, dip the brush in lacquer thinner and spin away in the chamber. Dip the brush frequently. 
You will want to have the action removed from the stock because lacquer thinner will play hob with your stock finish.

You haven't said what lengths you've gone to to ascertain the source of the problem so, let me suggest what I know that has been responsible for such issues.  As a rule, only very rarely does the problem prove to be head space.

Another thing is potential damage to the chamber wall.  Look for scratches on the case walls indicating chamber burrs.

What ammo are you using?  Have you tried chambering different ammo?  Are you using factory or reloads?  Improperly resized reloads, bullet lengths or seating can also be the source of problems. 

Come to that, I don't know just how significant the shoulder issue really is.  I know plenty of people who have shot Turk, German, Yugoslav and other original military ammo and also shot modern made commercial ammo, European and US made and no doubt encountered varying shoulder angles depending upon when the ammo was made, some of it dating back to the 1930s.   They have shot all that and not had any problems with any of it.

That's about all I can think of.  Let us know if any of this has helped.  If not, we'll rummage about and see what else we might come up with. If we can't supply the answer, maybe we can direct you to some place or somebody who can.

We aim to please!

Thanks for the update.  You have pretty well eliminated all other usual culprits.  Looking forward to your results with the gage.

Just be sure the bullet doesn't extend too far when chambering after you do that.

Sounds like you've got that covered then.