Since my p pump swap I've had this issue. Sometimes when I start it up, it runs a little rough, fuel pressure drops and it dies. I can play with the throttle and get it to clear up sometimes. Other times it dies and I have to crank it a lot to get it going again. I've also held the idle at around 1200 right after it starts and it won't do it. I live at 2k ft and it rarely does it. And when it does I can keep it going. But in the higher country, 5-7k its way worse. Once it gets past this hiccup it runs great. And idles smooth. It's only on startup.
Pump has been rebuilt. Lift pump is only 3yrs old. And I can hold 25psi at wot. Any ideas?

 

Sounds like you are loosing prime. Could be overflow valve, or more likely a return line with a very small air leak. Line is not under pressure so it doesn't always leak fuel out, but could be letting air in (and fuel back to tank). Pressurizing the system through the filler neck with a little shop air (no more then 10 psi) works well for finding leaks. Make note of any wet spots before and after.

 

I agree with gorms I think you are losing prime. Since you know your fuel pressure you must have gauges, if you have gauges you might have an isolator on the fuel line going to the gauge. The isolator has an Allen head bleed point and under the screw there should be a ball bearing. If you don't know it's there it's easy to take out the screw for bleeding and lose the ball bearing, the screw by itself will sort of seal but will let air in causing the kind of starts you are having.

 

I have a tork tek ofv. It shouldn't be that. Just remembered that I wasn't sure what to do with my injector return line when I did the swap. I tee'd it into the pump return line. Not sure if that's correct but it ran so I forgot about it. I don't have an isolator to my fp gauge. I'll try putting some air in the tank and see what happens. So let's say it is losing prime. Why the change with altitude? A very drastic change.

 

My guess about the change with altitude is the fact that atmospheric pressure decreases as altitude increases, with a small crack in a rubber fuel line there may be enough atmospheric pressure at 2000ft to keep it fairly well closed but at 5-7000ft the pressure is lower allowing the crack to open more. If you can't find it by pressurizing the tank I would change out the tank return line on spec.

 

Air is a whole bunch thinner up there, will find its way into small cracks much easier my guess. Cold temps amplify this usually also as the rubber contracts.