The likely cause is an automotive sender problem. Pressure senders compare the internal pressure in the fuel line to atmospheric pressure. The difference is what your gauge shows.
The problem is when a pressure sensor doesn't have a big enough static port and the air pressure inside the sensor is different than ambient pressure. sensor. This causes artificially low pressure readings in fast climbing aircraft.
When you descend, you probably see surprisingly high pressures. Same basic cause - the static port in the sensor isn't big enough, so the pressure inside the sensor is lower than ambient air, causing false high readings.
Once you're at cruising altitude for a few minutes, does pressure stabilizes at an appropriate reading? This is just more evidence that everything is OK with your system except the size of the static port on the sender.
