'65 gmc 1500 with 350 v8 missfire

[Harleyq]

Hi folks, been awhile since I had to seek superior knowledge, but here I am. The issue is she will start and run perfectly, just as she should, until it comes up to operating temp. Then all he'll breaks loose at low rpm, miss fires terribly with an occasional backfire and or die. If dies she starts right up. If at higher rpm the missfire will straighten out. I've checked vac hoses, changed fuel filter, checked timing. My question, will ignition modulator or hei coil cause these problems. This is a new one to me and spent too much time scratching my already balding head, can't afford any more hair loss....thank you in advance for any and all suggestions. ****** Dave

[James]

Did you pull the distributor cap off and check to see if the mechanical advance is not sticking. It should rotate a few degrees freely then spring back instantly. There should be almost no play in the relax position.