Check AND Double check the plug wires, Vs Fire order.
1 6 5 4 3 2
Rotation is clockwise.
A reversed wire will give exactly the symptoms you describe..
Test the Module, Take it to the local Auto zombie and get it tested for free..several times , to heat it up. And if you need a new one, have that tested also before you leave..they have an abysmal failure rate out of the box..When reinstalling new OR old, do not forget the heatsink Compound, Or you will be doing it again a week or so later..
While your in there, make sure nothing is hitting as the advance plate moves and shorting out the module or magnetic pickup.
Check the star magnet for metal shavings, this will give false triggers to the coil..
Check both advance units, mechanical and vacuum, be sure both are free and working not rusted and shut or the diaphragm blown. you can move the mechanical by hand, and you can add 5 to 10 In Hg to the vacuum canister and observe the movement.
Check the cap for carbon spray from the rotor pickup inside the cap.
Measure the coil, With your DVOM, set for R X 1 , Calibrate the meter, (cross the probes, ( for a 000 reading, or make note of errors) Measure the primary, put one probe on the "Batt" terminal, and the other on the "Tach" terminal, It should read less than an ohm, but more than 000 ohms..much outside that range , replace the coil.
Next measure the Secondary side of the coil, set your DVOM for R X 10k ohms..put one probe on the rotor pickup, and one on the "Batt" terminal. It should read 6,000 to 30,000 ohms, outside of this range, replace the coil.
This will not detect internal arc~over on the inductor core windings, you need an oscilloscope for that, so if you suspect it, replace the coil with a known good one.
Inspect the coil well for water or condensation.
On a stock engine with no cam, 5 to 15 In Hg Vacuum is pretty low, It should be around 17 to 20 In Hg AND STEADY. Look for leaks , get some carb cleaner and spray it around the manifold With the air cleaner on..(you'll get false triggers) everywhere it speeds up you have a leak. Inspect all the hoses, and PVC for leaks/malfunctions. If the Vacuum is rapidly fluctuating, you have some valve/seal/seat issues..
Do a compression test, you should have 125 to 150 all the way around, +/- 10% ..If ALL are low, you may have timing chain Issues..
Do a Static timing setup..#1 Compression ,TDC, BOTH intake and exhaust Valves closed, timing marks lined up, (If you can't coordinate these, you have timing chain issues)and Dizzy DEAD on #1 plug tower. If you have the first 3 OK but the Dizzy is off, reset it to #1.
Do a Fuel pump Volume and pressure test, if outside spec, replace the pump. Replace the Fuel filters.
If it sat some time, drain and replenish the fuel, it could be full of water, or lost it's punch..
Look for bends, or restrictions in the Exhaust system, if found replace the pipe.
If you have an EFE valve in the right header, (closes on a cold engine until it heats up then opens) BE sure it is not stuck closed. 78 shouldn't have one though..
Do these basics and you'll find the problem!