I recently installed a oil temp and pressure gauge, the sensors where both pluged into a sandwich plate and both of the gauge a electrical. after a week or so the oil pressure gauge needle started to twitch between 8-9 bar, a couple of times it went back to normal but now it sits in this broken state now, could the fault be the oil pressure sender on the sandwich plate or with the connection comming thru the fire wall, when the engine is off the gauge still twitches.

 

Mine twitches like that because the grounding of the sender unit is poor - mine is on a braided hose to the standard pressure switch and there's ptfe tape on the joints, so no surprise the grounding is bad. Improving the grounding with an earthing strap (as in household radiator pipes) around the sender unit reduces the twitching.

Hope that helps.