Suspect ignition coil - help please

[olddog]

I drove to town, and shut engine down for 45 minute.
Started engine and idle was fine. Started to go and engine started to miss fire. Rpm does not matter, but the more load you put on the engine the worse it got. On a free rev at WOT it acted like the key was turned off - no fire at all until throttle was closed. Stopped and checked fuel pressure (EFI) and it was on the money. Looked everything over could not find anything obvious. It started running better. Closed the hood and headed out of town.

Stopped at several lights and missfires got worse. As I got out of town and picked up speed it started running normal.

Let it set a few hours and did a repeat test. Ran fine cold. After setting for 25 minutes hot and restarting, I got the same missfire. Once is was up to highway speed, it started running ok.

Seems like something is breaking down when hot. My first suspect is the coil.

5.0 based 347 engine. It has Mallory distributor EFI computer controlled advance. The coil is Mallory as well. Coil looks like a Mallory Promaster E Series. Where its mounted I could not see any markings other than Mallory.

I measured 0.5 ohms across the primary and 8740 ohms from secondary to either primary. Engine is cold. Coil does not appear to be grounded. I read open from either primary to engine block and open from secondary to engine block. Coil is mounted to fiberglass fender well. I do not see any gound wires. I was a little surprised that I get the same reading from secondary to either primary terminal, but sense it is a coil, I guess it would be a continuous wire on each side.

Am I measuring this correctly?
What all should I check or better yet how should I procede from here?