Baffling Intermittent Issue

[Bikesmith01]

Hey Guys!
I'm having a tricky problem with my 1975 911s and I'm hoping you can help.
About a year ago, I started having two new issues simultaneously. One, the car was VERY hard to start if it had been sitting for a few days. It would take ten minutes of cranking before it would finally fire up. Then it would run very badly for 30 seconds. After 30 seconds or so it was like a switch was flipped. The car suddenly ran great.

Except for issue two: Sometimes, after running great for half an hour or an hour, the car would suddenly start coughing and sputtering. Sometimes it lasted only a couple seconds and the went back to running normally, sometimes it was on and off until getting home.

You all suggested testing fuel pressures. I did, which revealed problems with the fuel distributor and the warm-up regulator.

I had both rebuilt by someone recommended on this forum. I don't want to name names, and this isn't what this post is about, but oh man what a horrible experience. But after almost six months I did end up with both FD and the WUR correctly rebuilt.

The car started and ran like a champ, but still had the intermittent coughing and sputtering issue. Not every drive, but enough that there was clearly still something wrong.

I've, over the last year and a half, replaced the fuel injectors and ignition coil. I've adjusted ignition timing and dwell. I've replaced spark plugs. I've replaced the fuel filter. None of that was because of this issue. I just mention it so you know I've given attention to those things.

I wondered if the coughing/sputtering issue could have been caused by water accumulation at the bottom of the gas tank. Before I got it the car had sat for several years, so I thought maybe the gas had separated. So I drained and dried the tank and refilled with fresh ethanol-free. After doing that the car actually went a couple months without a single issue.

But recently, just after a gas fill-up, it started doing it again. Even more frequently than before, but still intermittent enough to make diagnosis really problematic.

A stethoscope on the fuel pump revealed it sounding rough. I wondered if it could be cutting out intermittently. So I replaced it. Just finished that today.

The car started up instantly and ran great for 10 minutes, then coughed and sputtered. D'oh!

I'm at a loss for what to try next. I'm tempted to try draining, drying, and refilling the tank again, but I know that's a long shot.

I also am wondering about the fuel hose that supplies the pump. Just before it gets to the pump (which is at the rear of the car) it does a vertical loop, like the loop on a roller coaster. I'm wondering if there could be an air pocket at the top of that causing trouble. I would think the pump would suck hard enough to clear an air pocket, but maybe not. I might try bleeding that section of hose, but again I feel like that's grasping at straws.

So, that's the situation. While I'm measuring or checking any system, everything is normal. Fuel pressures are spot on. Pump is running great. When it's not having the issue, the car is running beautifully. But then I get these fleeting, intermittent moments of the car running like crap. I'm tearing my hair out trying to diagnose this.

Any thoughts?
Thanks, Jon