Karcher 411A pressure pulsing when spraying ?

When im using the pressure washer it pulses when i depress the trigger. Its not pulsing when the trigger is not depressed.
Ive removed the case and watched it run. There is a pressure switch that cuts the power to the motor when the trigger isnt pressed to stop the pump destroying its self. This mechanical pressure switch is pulsing in and out as is the pressure is getting too high for the machine.
So, im looking for any advise on why this pressure switch should pulse when the trigger is pressed.
If i hold the switch forward with a screwdriver it runs fine!
Can anyone help please!