I had a progressively lowering fuel pressure on my O-360. After a lot of troubleshooting, it turned out to be a failing sender. The senders are not intended to be mounted hard to anything that vibrates (like and engine), they wear. They should be remoted to a non/less vibrating mounting with a flex line. I found when I disassembled my failing sender, the wiper which runs on the rheostat wire coil had worn so much that it had a much large effective contact width, and it shorted out a number of the rheostat wires, causing a reading which was false, and changing as the wiper wore more. It would still make an electrical connection, and send the signal for a pressure, but it was inconsistent and wrong.
Try changing out the sender. If you can mount it remotely, that will help preserve it.