Peter, lots of info on this web site about your problems use the search box with key words.
Overheating on a diesel engine is us ally water flow, you may think there is lots of water but how do you know!
Go through the process of elimination, back flush with a hose pipe connecting from weed filter back, a good pressure is needed.
Then you start on impeller housings, pipes and heat exchanger as these are a common source of blockage. Bits off a broken impell or get stuck in heatexchanger.
Exhaust elbow is the final issue I know of, a build up of "crud"reduces the flow, but start with the first possibilities.
Bad running could be fuel as this is another common issue, look at the pinch points of fuel delivery, filters, tank pick up pipe, fuel switch as this is a gas switch really with very small holes for fuel to run through, see photo.
If you are in Europe remember, they put 10% bio fuel in now and this is like home from home for the diesel bug. Put fuel set in each time you fill up.
Come back after you have had a look at above.
Hope this helps
Odysseus