Harry,
Sorry I am not participating more often, this is a very busy time for us.
With the action closed take a look at you vertical locking bolt. The vertical bolt should be at least flush with the top of the sliding bolt but will probably be slightly above it.
Open the action and look at the locking pin on the rear of the link. The link drops with the lever. The locking bolt is spring loaded so push it in against the spring tension.
We have another thread on this bolt somewhere here on the Beartooth forum. I measured this for another guy, I’ll look for the thread. It ended up we had a similar problem with a Model 94AE in 30-30. WE pushed out the horizontal retaining pin and removed the locking pin and deepened the slot a few thousandth’s. This allowed the locking pin to come back just a bit and stopped the unlocking on firing problem. The hole thing took about 15 minutes.
The important thing too look at, and you said your loads were less than maximum is whether or not the lever stings your hand - I term this lever-kick. If you are not getting the sting of lever-kick you probably do not have a serious problem but rather a minor adjustment problem.
I wish we had tried the 30-30 with some low pressure loads to see if the action tried to unlock. It is really the recoil , I believe which causes our lever hand to unlock the lever but I never tried to prove that. I’ll watch this thread closer and see what you find out.
There should just be a moments hesitation as you unlock the action with an unloaded rifle, do you feel this as the locking pin on the link comes free?