Lurching problem and I'm out of ideas.

This could be a 12 paragraph explanation, but I’ll try and keep it short. I’ve been problem solving this for months and can’t figure it out.

My question: Is it possible for a rear end to cause this? Can a torque converter be in bad shape and not leave metal shavings in the trans pan?

Vehicle: LA360 (Lumpy cam 235/249, EFI, MSD ), A999 (don’t know if it’s lockup), 8 3/4 Suregrip

Problem: When car has been warmed up for a while and driven for 1hr plus, I get a hard jerking/lurching between 1200-1300rpm when coasting down (no braking or accel). The jerking stops when rpm drops to around 1100, and disappears if I accelerate or brake.

What I've tried: I’ve taken off the intake manifold and completely redone all gaskets and checked for vacuum leaks, I’ve switched to an adjustable PCV valve to ensure it’s not a low rpm vacuum issue with tight overlap cam, I’ve tried different timing curves, CD boxes, and ignition coils. I switched from Sniper EFI to a carb and back to EFI. Fuel pressure is solid and I’ve checked every part of that system. Inspected cam and lifters too.
The engine is in very good (almost new) condition and the problem usually only starts after driving for a long time, so I don’t suspect it’s the valves. But I will do a leakdown test soon?

I’ve verified the kickdown lever activates at a 1-1 ratio with the throttle lever. I’ve taken it off, tightened it, loosened it.

I’ve gone to two transmission shops and both have said they don’t think its the trans. One shop took the pan off and said it was clean. Oil smells/looks good. I wen't to the best transmission shop in town and they said I had a "funny one" and couldn't figure it out. I kinda suspect neither shops were interested in dealing with a modified old car.

This issue did not exist for the first 5000 miles of driving, and started occurring after some hard hits.

Any ideas welcome!
Thank you for your time.
Ben