You're right about "jammer" being vague. Its been awhile but I would be shooting it and the hinged action would stick in the upright position (like its supposed to do then you expend the magzine after the last round is fired). Then I would observe one of two things:
1) The expended shell casing would be stuck and to eject it I would have to pull the action back a little bit to get it to eject and then let the action go and the next round would load.
2) The other thing that happens is that while the expended shell casing has been ejected, the next round would be maybe a 1/4 of the way in the breech but somehow cocked at a slight angle so it wouldn't go in all the way. Again to unjam the gun I would have to pull the action back a little until the bullet was free and able to reposition itself in a straight line with the breech and then I'd let go of the action and the bullet would load.
I cleaned and oiled the action with Mil-tec1 numerous times but all to no avail. My guess is that a gunsmith would probably be able to spot some binding points in the action that need to be sanded down, etc.
You do mention fact that jamming was typical of gun when brand new, heck, I've had gun for 20 years and I'll bet I have shot no more than 150 rounds through the thing in all of that time due to jamming problem.
Jim