Hot start problem, Checking opinions before I buy a new battery Options

[Highland]

My car starts easily when cold, but hot it has been a struggle. I put a variable resistor on the CHT as an experiment without success. Engine's not even catching.

Plugs look good, 123 distributor, blue coil, valves adjusted, runs fantastic once started.

I am using a 2 year old motorcycle AGM battery(which has been working prior to 123 distributor install). Open voltage measures at 12.6V.

I stuck a lead on the coil while cranking and voltage drops to around 10V and will bounce down to 0 occasionally if I crank for more than a few seconds. With ignition only the coil is at about 12.3V.

The blue coil is an NOS coil that I had. It's probably over 30 years old but resistance is good when cold. I did not think to measure the resistance hot, but would it make a difference? Does the insulation breakdown over time even if resistance is good?

Thinking my little battery doesn't have enough power to crank and fire the coil. Any opinions? Cranking is strong.

[porschetub]

My car starts easily when cold, but hot it has been a struggle. I put a variable resistor on the CHT as an experiment without success. Engine's not even catching.

Plugs look good, 123 distributor, blue coil, valves adjusted, runs fantastic once started.

I am using a 2 year old motorcycle AGM battery(which has been working prior to 123 distributor install). Open voltage measures at 12.6V.

I stuck a lead on the coil while cranking and voltage drops to around 10V and will bounce down to 0 occasionally if I crank for more than a few seconds. With ignition only the coil is at about 12.3V.

The blue coil is an NOS coil that I had. It's probably over 30 years old but resistance is good when cold. I did not think to measure the resistance hot, but would it make a difference? Does the insulation breakdown over time even if resistance is good?

Thinking my little battery doesn't have enough power to crank and fire the coil. Any opinions? Cranking is strong.

I'am running an AGM which is commonly used in my country as a speedway car battery ,only issue I had with it is when they get too flat they need to be woken up by a low amperage charge,this isn't often as it has a very low discharge rate even when not used.
One other issue I had was a non-matching regulator which appeared to confuse my reconed altenator so the charging was not consistant to load ,changed to NOS Bosch regulator and the issue went away .
I had a original coil in one of my VW buses that would break down after about 15mins or less if a hot day so maybe you should start there ?.
If you have good cranking speed I don't think you would have a voltage issue @ the coil unless you do have a suspect one.
I just checked my AGM and it is 12.97 volts sitting out of the car ,I topped up the charge around 2 months ago so maybe a full charge on yours may help.
Good luck.