Wandering oil temp readings?

My vans oil temp gauge gives wandering read outs. It seems to work while engine is warming up then when temps get to about 180 needle will go to 50 then up to 100 then to 0. I replaced sending unit and the gauge. Checked for loose ground. Havn't followed the wiring looking for chaffing. The needle takes 2 or 3 seconds to go up or down. Anyone else has this condition? Thanks