Hi, ok I see this allot.
The problem is the water pump drop the gear case and check it. They need to be replaced every two to three years as not to get into overheat trouble. You may take it out and think it looks ok but replace it!.
You see the impeller provides water with pressure to the power head for cooling. The water /pressure needs to be suficiante enough to fill the whole cooling system cavity of the block in order to pull the heat away and operate the thermostat.
On most overheat I will replace the thermostat anyway for insurance.
Hope this helps you out,
There is a thermostat cover on the top/back of the head. on some models they didn't use the thermostats on the 35. If the cover slide into the head with deep sides on it (insead of just a flat cover) it will not use one.
If the cover slides into the head and has deep sides built into the cover that slide into the head then it doesn't need a therm. Is how it should have read, sorry. I
Now if your head has a one piece water jacket cover along the whole head with lots of bolts holding it on and has a bump on top where a thermostat would be underneath it's there. You'd be able to see a part of the therm looking throw a hole on the inside of the head possibly.