Dave,
<<It seems to occur when there is an internal leak that allows the high pressure side to slowly leak across to the low pressure side,>>
When the spool valve is installed correctly the down circuit is essentially open to atmospheric pressure via the reservoir.  Because of this, any leakage from the high to low side cannot build up pressure on the low circuit.  If the spool valve is in backwards, pressure can build as a result of a leak up to the setting of the relief valve which will be higher than that of the pressure switch.
Chris Zavatson
N91CZ
360std