Less intermittent as time goes by..
The first time 60 days ago I was going up a grade and had about 75 miles left on one tank and the other was very low. The truck started sputtering like it was out of fuel so I did a U-turn and filled up both tanks. Problem went away for 10 days and again, going up hill it started sputtering. I pulled off to the side of the freeway for a minute and took off without a problem for 10 days. Then soon after that the same two hills have always caused a recurrence.
Changed the fuel filter and no difference.
I took it to my trusted and very good mechanic and he tested everything and found no problems. He suggested tank delamination so I switched to the other tank and went on. Same problem with both tanks.
Next he suggested that I remove my four position chip. I did that and the problem went away going up the grade on the hwy but on the same hill that the problem started on it lost power and could only hit 45MPH. With the chip installed it would max out at 25MPH.
What's next to look at?
TIA,
Vince

 

I would check fuel pressure while driving under a load.I bet it is falling off.

 

Make sure the tin nut is on the back of the IPR.

 

How do I test FP while driving and where is the IPR?
And thanks!

 

I second the ipr. It is behind below and to the right of the hpop.

 

Got it thanks!

 

Check the FPR screen in the fuel bowl, it might be clogged.

 

Link to FPR screen DIY?

 

Where is a good source for IPR? And what is the difference with or without filter?

 

As far as the FPR screen, if I remember right it on the inside of the fuel bowl on the drivers side. I've had electric fuel so long I forget the procedure but some say take a Q tip without the cotton on it a scrape it clean.

Riffraff has IPR's and I bought one from TerminatorEngineering once.

 

It looks like it was the IPR valve!
I Rockauto'd a genuine Motocraft for 150.00 and I can't make the truck do anything stupid after 10 minutes of hard driving.
Whoopie! And thanks for the troubleshooting!

 

Not the IPR valve. It misfires on the same long grades. Not nearly as bad though. I can maintain 45MPH instead of 25MPH. So the new IPR made it better but the root cause is still there.
What's next?

 

I'd check this first. Stock hpop? Any CEL?

 

Will do. Stock, no CEL or codes thrown.
What would cause low FP under load?

 

A weak fuel pump.