Engine/Transmission (1998.5 - 2002) Bad lift pump out of the box?

[Weidner]

Truck fp started to go lower and lower over a couple month long period was 15 and went down to a max of 7 at idle and cruising speeds. Then (not sure if this has something to do with it but the outside temps have been really cold 0 degrees and below at night) it started to get down to 3 and then zero and bounce back up to 5-8 while driving at highway speeds. I changed the filter, same thing happening. I changed the lift pump, then I could bring it down to 5 at wot and 8 at cruising speeds I know that is low, but it was better than before. That lasted a day, today it went to zero at highway speeds and then would pop back up to 3-6 and 8 at idle, in town below 50 mph I am getting 6-8psi. Is this a bad pump or do you think its my electronic gauge sender or any other thinks I should look for? It is cold out and I am not in the mood to work on our only winter vehicle. I am going to try a mechanical test gauge tonight, but that will be under hood... any advise or thoughts?