Bram,
It does sound as if the warm up regulator is at fault but I would personally want to check the control and system pressures before replacing an expensive regulator, especially as failure is rare.
You won`t see any immediate effect from disconnecting the wiring as it only operates a heater element which causes the the regulator to slowly change the control pressure as the engine warms up. For a very good explanation of the system`s function Click Here,
One possibility is a blocked return pipe, causing excessive pressure to build in the system.
Colin.