I have a 1987 Formula 223 with a 350 merc mag (approx. 390 hrs) and
I've replaced the thermostat and impeller because it was overheating to
over 200 degrees. The Thermostat did not solve the problem so I changed
the impeller in the outdrive and the problem is still there.

It runs at about 160degrees at idle and then at high speed (40/50 mph)
its still comes down to 160 degrees ... but at the middle speeds, with
trim lowered, making turns, slowing down etc temp will reach
200degrees? my oil is pumping at approx 30lbs ... I checked the oil,
the gauge seems to work fine ( i checked the ground to it and it
works?) ... wondering what temp the boat should run at ( i thought
approx 160degrees)? any ideas or suggestions? much appreciated.