Based upon your description, I would be thinking of a problem in the sensor. If the pressure stays high throughout the flight, but after shut down and restart, the same flying conditions produce a lower pressure, something is likely hanging up in the sensor. It is really just a diaphragm and a spring.
The Vernatherm can't really increase pressure at the sensor as it is downstream. Hanging up in pressure relief mechanism is possible, given your symptoms, but much less likely than the sensor hanging up.
Oil viscosity needs to be considered. Cold oil is thicker and can increase the oil pressure if the relief cannot shed enough oil volume to meet the target pressure. However, that condition should be gone once oil temps are around 100* and this is not consistent with your symptoms. I would be looking for something "hanging up" if understand your symptoms correctly.
It would not be abnormal for your pressure to be 95 (high RPMs) when cold and then slowly reduce to 85 (high RPM) when warm. It is abnormal for your pressure to be 95 on the entire flight after a cold start, but 85 on the next flight with a warm start. When troubleshooting, always compare pressures at a common RPM. It is normal for engines to produce different pressures at different RPMs.
Good luck,
Larry