What a pain!
First, my results on a 95 854T, engine cold. Engine switched on, pressure at 30+ but falling rapidly. Started, still cold - 38psi at idle. After 5 minutes, engine warm, psi 38 and steady.
A quick rev, moves the pressure to 42 and then down to 36 at sustained rpm about 2K.
At steady idle or rpm, pressure remains constant. Your fuel pump or regulator may be getting tired.

Pain part:
My gauge is too long at the schrader end. I had to slide the throttle cable housing out of the way, which means sliding the IAC also, and taking the bolt out of the vacuum tree, only to put it back. The tree was in the way of the 3rd bolt on the throttle housing.
The AC compressor started to click on/off, changing the pressure from 38 to 36 at idle until the IAC corrected it.

Klaus

--
I still miss my 164