Base upon the speeds you mentioned, my first thoughts were a plugged cooling system or the head gasket. If everything is working good on the water system, I would guess the head gasket. Going slow, water is going slow and has a chance to heat up. When you speed up, the water speeds up and doesn't have time to heat up. It stays cooler so the engine stays cooler. Pull the spark plugs and check your cylinder compression and see what color the spark plug electrodes are.

Head gaskets are not expensive, but they can be a bear to change. Especially if you check everything out and do the other little things that could be done "as long as you have it apart".

Good Luck with it.

Steve