I have closed cooling system.When cruising at 3500rpm my temp guage slowly starts rising, never overheats but you can tell it's getting hot. When I come off plane the temp skyrockets over 200 degrees.I've replaced the impellerer, changed the t-stat. I noticed a slight drips of coolant out of the exhaust manifold water inlet seals that go away when the rpm's go up, I know I need to replace those. Could this be my only problem or should I consider flushing the exchanger? The engine's has slightly less than 700 hours. Thanks for all your help.