The rifle is a Winchester Mod. Super Shadow. The bolt lift is heavy or stiff when opened with the safety in the fire (forward) position, but opens easily with no drag or heaviness when the safety is in the middle positon (on safety but bolt not locked). It does this with an empty chamber as well as when a cartridge (unfired)is in the chamber, but is more pronounced when a cartridge is in the chamber. Is this normal???, and what causes this??

It's normal if it's not severe. The M70's load quite a bit of tension from the firing pin spring on closing so it would still be there on opening.

Have you compared it to other M70's?

Make sure there is some lube on your bolt lugs.

Most likely it's the cocking piece that's rubbing on the bolt body. Since the cocking piece doesn't have a true helix from the factory, even though it's supposed to, it will cause excessive friction when working the bolt.
When you place the saftey on the middle, or full safe position, you relieve the tension from the firing pin spring, which makes it feel smoother.

A little polishing on the bolt body and the nose of the cocking piece may cure the problem, but if it doesn't, I'd highly recommend contacting a local smith to fix the problem.
Do not alter the angle on the cocking piece that's contacting the sear.

And to add to the prior comment, ensure that there is adequate lubrication on the bolt lugs.

One other item that frequently gets overlooked is the polishing of the threads on the bolt shroud. A little wire wheel work will go a long ways towards smoothing that up, as well as adequate lube.

Assuming this is with the rifle cocked, it is a result of the upward pressure on the cocking piece from the sear. All rifles which feature an angled contact between the cocking piece and the sear will exhibit this to some degree but it can be more severe on some individual rifles even of the same make. This may be due to a variation of the angle on the cocking piece or sear or both. It may be due to the amount of contact between the two. Other possible causes could be related to the fit of the threads of the bolt shroud within the bolt and the amount of friction between the bolt body and receiver. I've always felt that, of the overriding sear type triggers, those work best which feature a vertical contact between the cocking piece and the sear (the Timney trigger for the M98 for instance). This doesn't tend to lift the rear of the bolt up to the same extent as those with an angled contact.
The cure? I honestly can't say with any degree of certainty simply because there are too many possibilities. Regards, Bill.

I had the exact same problem with my mod 70 SS 375. I took it to a smith and had him take it apart and clean/polish averything. I have hunted it quite a lot over the last 4 years and it had accumulated some "Gunk" in obscure places. Now smooth as silk. Good hunting. "D"

I have three Mod 70's

A factory gun I paid 500 dollars for

An FN SPR semi-custom I paid 1000 dollars for

And a full-blown custom Classic I had custom-built for 3000 dollars

And they all exhibit the same thing, almost effortless with the safety in the middle position,
And a fair amount of effort with the safety in the fire position.

I would concur that this is normal, if it is not excessive...
Perhaps you could compare with another one? or two.

Good Shooting,
marc