Roy Page wrote:

On the flight home, we noticed on long climbs that the fuel pressure fell
off from an indicated mid scale reading to about one third scale.

snip

A few months passed and then occasionally I noticed a slight fall off in
fuel pressure when climbing.
I concluded that the mechanical engine driven fuel pump must be getting
tired and, to be safe, replaced it with a new pump three weeks ago.
No change, the fuel pressure still falls off when climbing and returns to
mid scale in level cruise.

snip

Our PA28-180 has a similar fall off in fuel pressure when climbing.

I don't know what causes it, but the PA28-180 I previously owned did this, too.
First time I saw it, I made a precautionary landing, no trouble found. Like you,
I went over everything and never found any fault. Eventually I concluded it was
just a peculiarity of the model and was benign. If you find a way to fix it, let
us all know.

Dave