Strange running issue, Fuel tank vent? Options

[sdoolin]

73 2.0L, built to 2056 running dual Dellortos. I have just over 3k miles on the engine since the rebuild, and it has been running strong, and I have been loving it. Went to pick up my wife from the airport last week and about 30 miles into the trip, the car started running poorly. Really poorly. Had been all freeway miles, and I was running at 80+ MPH. Car had been running fAntAstic!

Felt like it was running on 3 cylinders, and running out of gas. No strange/unusual mechanical noises before poor running, or during poor running.

Completed the next 5(ish) miles to the airport and strangely, once I slowed (exited the freeway) the engine picked back up and began to run normally again. Made the 40(ish) mile trip home, running poorly the entire way. Pulled into the garage, shut the car off and heard the STRANGEST noise from in front of me. Sounded like the gas was boiling in the tank.

Quickly opened the frunk and removed the gas cap. Large sucking sound on gas cap removal. So, I am thinking that my tank was not venting properly, not allowing proper fuel to get to the carbs? I have put another 300 miles on the car since this event - no more issues.

I am running a 3 PSI pump, with a return line from the carbs.

Anyone else ever experience this (or similar)?