1994 Honda civic loses oil pressure after warm up. Starts to chatter very faintly. pressure gauge looks good randomly, but then will drop when revving it goes down and then up and then down . Already replaced the head gasket, oil pump, rod bearings, thermostat misc hoses and gaskets, as well as checked screens filters. Crank bearings look great and same with the crank shaft very stumped as to what could be wrong.