Again GUESSING, but possibly the station is getting fuel from a different supplier, or the same supplier with different additives or the temperature where you live has been like it has been here the last month (MISSOURIABLE !).
Fuel has changed substantially in the last 40 years.
If I am right, the problem may disappear completely once the weather becomes much cooler.
The half fuel tank is NOT helping, but I understand your reasoning.
Do you get a WHOOOSH when you remove the fuel cap? Re-reading your answer, probably not. If your tank vented through the cap or does it have a tank vent? If the tank is vented, the cap should seal.
On a totally different vehicle (Dad had a 1981 Winnebago built on 454 Chevrolet chassis), about 1985 started having this issue (gas changed). When Dad stopped to fill the tank, one needed to stand around the corner from the fuel cap, and release the cap. If one did not hold on to the cap, the inner tank pressure would blow the cap 25 feet in the air !!! Even though this was a 1981, the heavy duty trucks had less smog emission equipment than passenger or light duty trucks. I completely solved the issue by junk-yarding THREE charcoal cannisters which I then hooked up to engine vacuum with appropriate valves just as if equipped originally. The three cannisters were required because the 454 had a 75 gallon tank, and the cannisters came from vehicles with 20~25 gallon tanks.
He had absolutely NO problems until he did have problems; and the modification solved the problem.
Try my solution in my first post. It won't cost you a penny; and it may work.
Reference:
Hard starting and rough running hot engineJon
