Runs perfectly fine at idle and slow speeds. Once I get it up to 30mph to 60mph it immediately starts to overheat with warning buzzer going off. Temp gauge goes to 250. Once I bring it back down to idle it cools down within a few minutes. I have taken it to a shop who unfortunately was not by the water so they could not test drive it. They replaced the thermostat and impellers on it. It did not solve the problem. They checked the block and it is not cracked and boat has always been stored in indoor heating year round. I have been told it cannot be the heads or block because it only overheats at high speeds. I need suggestions. Can someone please help?

Start by checking that hose that runs from the drive to the block is not kinked or loose. I had this happen once on an Alpha drive. Found that the hose had not been installed correctly at dealer / factory prep. The hose would blow off at high speed, but got enough water to the system at low speed to keep it from overheating at low speeds or idle.

I appreciate your response but if that were the case wouldn't there be water pumping into the boat under the engine? My boat takes on no water at all & I would think that if there were some form of a leak like that I would notice. Maybe I am wrong though. Again I really appreciate all thoughts on this.

You may also want to check we're the water enters from the transom. Pull the hose sometimes that water inlet which is plastic can collapse or can catch debris. Also pull,the plugs at the bottom of the block.

I had a similar situation where a barnacle had grown inside the water inlet on the outdrive, then came detached and lodged in the transfer hose further along the water inlet passage way.

The engine would heat up at cruising speeds and cool down at idle speeds.

Another thing that it could be is a cracked inlet hose in the bell housing/transom assembly area.

Impeller ..... Stuck Thermostat .....or the thru the transom fitting (very common) ......I have seen em all .... the fitting is plastic and corrosion between the fitting and the aluminum grows and collapses the fitting and restricts water flow ... Part # 16 but on the other end .... when they replaced the impeller did they flush the lines going to the oil cooler ? Trash could be restricting water flow thru the oil cooler/motor .....

How old are the exhaust manifolds ....restricted water thru the manifolds can do it .....

Have you checked the strainers to be sure there is no debris blocking the system?

Also, check to be sure that the water pump intake hose is not (collapsing) sucking itself closed at higher speeds. This is common when the hose gets old and the spring that helps it maintain its shape rusts away and there is nothing to support the outer walls of the hose and it gets sucked closed.

It's likely a collapsed plastic compression fitting where the water supply hose joins the inner transom plate. Take a 7/16" wrench and remove the flange that bolts to the engine side of the transom plate and tell us what you see. If it's all corroded and you cant stick your pinky finger in the hole, the corrosion has grown and caused the plastic expander to collapse. there is an outfit in Lousiana that makes a repair kit with a stainless steel fitting and new hose for this specific problem. surf here http://www.amarket.com/distributor1.HTM
Best Regards
SmokinDave