Troubleshooting 350 flatspot- need forum help

[marina blue bob]

Need your help ---
1. I've got a flat spot where the engine just dies (like you shut off the key) when trying to accelerate above 2000 rpm under load.
2. It starts fine, revs fine just sitting still with no load. Do you think
it's carb or distributor related ?
3. Been thinking it may be stuck mech advance. Trying to decide where to start diagnosing first.

Appreciate anyone's help.

Details:
67 K20
rebuilt Chevy 350
Holley Truck Avenger 4bbl - 670cfm
Mallory HEI distr