[HSSS427]

Sounds like a good mystery. Just a WAG at someting - is the distributor cap on correctly ie the tab on the cap is engaged with the distributor body?

Also, sound like you had the isssue, pulled the distributor out and then reinstalled it and got the same result, but might not hurt to pull it out and re-install from scratch.

What I do is get the crank turned so that #1 is at 20 deg before TDC. Install the distributor so that I've got the rotor pointing about to where I want #1 connection to be, and then eyeball it so that the rotor and #1 tower are aligned. So now it's set to basically 20 deg advance. Then start it and adjust from there.

It is interesting that something appears to be changing while the engine is off like the advance springs and weights are staying in the advanced position at idle and then when it cools off are pulling the weights back to a less advanced position. You might try it with the heavy springs installed vs a lighter set and see what that does for you.

Other than that, you might see if you could round up another distributor to test it with - might be someone around the area with one laying around you could borrow.

Good luck - believe me I know these "mystery" issues can be frustrating!
Scott