There is a mechanical, adjustable back pressure valve on the brine output. That is controlled by the knob on the panel.
While the logic board does monitor the pressure, but it has no control over it. All it does is shut down power to the pump if the pressure is too high.
The most likely causes are on the pump side with water supply, check valves, or motor speed. A bad back pressure valve COULD cause pressure problems, but I doubt very much they would vary with time. Those valves are very simple.
On the hard at LMC,
Fort Lauderdale, FL