489 Temp weird temp issue
 

40 hrs on 489, new temp gauge, new sender, new 160 deg thermostat/gaskets/etc,new impeller,new circ pump. started seeing an issue last season where needle will only move slightly up to 120 deg, then it stays there as the boat is driven. Come off plane, gauge spikes up to 180-200, then quickly drops back to 120. Temp gun reads 160 the thermostat housing. Heads/valve covers/intake are hot as they should be, risers cool like they should be. Only part I haven't replaced is the thermostat housing. This orig was a saltwater boat. Could their be an internal leak causing these weird temp readings.