You might have built up a rust sump in the barrels. Take the wee drain plugs out of the bottom of the barrels ONE AT A TIME!!! (replace them each in turn) and run the motor for a wee while to pump the water.. don't run it for long as the cooolant flow will be disturbed, and your exhaust will be under-cooled.
Often, the wee holes are in a rust sump, and need to be poked with wire to flush out the rust. That might help, though I am still puzzled as to why you are overheating.
Try this too.... take out the thermostat and check that it is opening.... it is under the cover at the fwd end of the manifold. Just boil the thermostat in water and you will see it open. If it does not open, and you don't have another one, don't despair... just bash the middle out of the thermostat to leave the outer seal ring, and re-assemble without the thermostat mechanism.
This will have the effect of allowing cooling circulation all the time. I really cannot see how you could be overheating then. You should be over-cooling then, and the motor will take that for a fair while... certainly better than overheating.
Are you pumping enough water from your coolant pump? Have you checked that enough comes out with the exhaust?
I have had a love/hate relationship with that MD17C over the years. The motor is very strong indeed when set up right and maintained, but the price of spare parts is absolutely shocking.
I went to the west of Scotland earlier this month, a round trip of 180 miles, about 160 of it motoring. An engine mount slacked off a wee bit and had to be tightened, but the motor ran fine, and used very little oil. I took off the engine panels to allow instant checking and access, the idea being that it would encourage inspection and warn me of impending faults. The engine room is like a wee submarine... panels and lights and fuel pump. It's a bit noisy, but so be it.