Erratic timing during vacuum advance

First let me say I have not owned a timing light since 1978 but will pick up a basic one today.

Did initial timing by ear and set it so it idled nice and sounded good. lol

Motor is a '74 mild build 318 with stock bottom, KB pistons, Mopar purple cam (dont know specs, bought the motor pan to carb complete from a third party) stock 74 318 heads with over size 360 valve job, gasket match porting and blending, 340 exhaust manifold and 2.5" duals, performer intake and 600 cfm Holley street avenger carb. New fuel system from tank pickup to carb with fuel pressure gauge.

Ignition is Petronix flamethrower HEI with matching flamethrower coil. Distributor is a re-manufactured stock Mopar electronic ignition unit, MSD wires NGK plugs gapped at.045

When I had the car on a trailer for exhaust install, I stopped by a friends and we checked the timing with his digital light. as I recall initial timing is 18*. I was in the car throttling up to 2500 rpm and my friend said the timing was jumping all over the place when the advance kicked in. I could feel the motor falling off as I gave it more gas.

I'm no mechanic by any means, any ideas why the timing jumps around like that during advance?

Thanks!