Since I see that the root cause issue you are trying to solve is related to timing under load:
1) How long have you had the car? Are you certain it's not bad gas?
2) Have you done a vacuum test? If you are reading low vacuum at idle, it's time to find the leak. The gaskets, a manifold crack, or even the valves being leaky could cause this issue
If you have the intake off, take the valve covers off and:
- See if you have any play in the rocker arms
- Loosen the rocker arms so the valves are shut (tap the springs with the mallet) and redo the vacuum test to see if the leak is at the valves.
See my experience: Issue Solved - engine bucking/surging at partial throttle
3) have you done a compression test?
4) Do you have a real oil pressure gauge? Low oil pressure could be a sign of really worn bearings which could lead to knocking. The condition of your bottom end is not something you can confirm without pulling the oil pan off but confirming your oil pressure is a good indicator to see how worn out your engine may actually be.
Keep in mind that even though you think it's a "fresh" engine b/c it only has 85K mi on it, this was an engine architecture that was designed in the 60s and back then, they did not expect an engine to last this long. This is no honda and it's been my experience that my 5.0 engines were due for a rebuild (low compression, felt down on power) by 95K mi even though one of them was owned by an old man. Sure, he might have paid for maintenance whenever someone told him he should have something done but he probably also forgot to do some maintenance.
Your already most of the way to 100K and a harmonic balancer that has slipped slightly is a very real possibility. IMO, Rayo's suggstion makes sense.