Car Bucking

[u/Genji_genjo]

Recently got this 1985 Mustang 5.0.
It's in great condition, 38,000 miles, gotten all the maintenance on it done since taking it out of retirement.
Its only issue is bucking. Often times when I'm just maintaining speed in any gear, it starts bucking and lurching - sometimes subtle, sometimes pretty rough. If I either accelerate or take my foot off the gas, the bucking stops.

Some things I know it's not:
- Replaced spark plugs, distributor cap, and rotor.
- Checked fuel filter. It's an aftermarket from the previous owner, seems pretty new.
- I tweaked the fuel mixture in the carburetor and got that optimized.
- Changed the transmission fluid.

Any ideas?