The inlet to the gauge has a restrictor to dampen the movement to the pulses from the induction.
If you disconnect the pipe at the back you can put a screwdriver into the hole and undo the restrictor screw about 1/8th of a turn. Then reconnect the tube and run the engine and you should find that the problem is fixed. If the needle vibrates then tighten the screw again slightly.
If this doesn't fix it then it will need to be professionally dismantled and remantled together with payment of the normal £x00.
ZA