A strong possibility for such intermittent cooling water flow stoppage at high rpm is an exhaust manifold that is beginning to fail.
Cast iron when it cools in its foundry mold forms 'stratifications' in the metal structure; the metal essentially forms a stratified structure similar to an onion. Once rusting eventually begins, accelerated by storing the engine 'dry', the stratifications begin to be pushed apart by the formation of ferric (red) rust and the result is the formation of 'slab rust'. The small slabs fall off and accumulate in the bottom of the water passages of the exhaust header .... until max. velocity of cooling water disturbs the slabs and they begin to 'move'. Most small diesel exhaust manifolds have both their water inlet and outlet on the 'top' of the manifold .... the particles essentially stay inside the manifold. Typically a few small 'slabs' of rust 1/5 the size of the outlet nozzle of the manifold is enough for the particles to 'bridge' together and fully shut down the flow (at max. flow) at the outlet nozzle of the manifold .... only to fall back down inside the manifold when the engine is shut down.
If you are 'brave' here is how to test for rust slabs inside the HOT manifold .... dont get burned: Run the engine until HOT and the flow decreasses, keep the engine runnning and pull off the hose at the outlet of the manifold --- if there is flow then the blockage isnt in the manifold; if there isnt flow remove the hose at the intlet to verify flow - if flow, its the manifold that is 'slabbing closed'. You can do this by applying small 'cock valves' to analyze with less risk of getting burned/scalded or you can install a temporary 'hose jumper' across the manifold - but can only run 'briefly'.
The alternative is to remove the exhaust manifold from a cold engine, turn it upside down and shake out the small 'slabs'; you may need to reach inside the manifold with a stiff wire to break up the larger 'slabs' (you may even find broken pieces of pump impeller along with the slabs). Then, pressure test the manifold to verify its integrity.
To prevent such 'slab' formation: always run the engine at max. for a moderately long period (the longer the better) to help develop black/ferrous rust, which is protective; never 'drain' the engine for long term storage - always use an antifreeze with rust inhibitors.
Once a manifold starts to develop 'slab rust' the danger is that eventually there will develop a pin hole between the water side and the gas side ... and you should perform periodic pressure testing of the manifold to be sure that there arent developed 'pin holes'.
Get yourself an infra-red indirect read thermometer and profile the entire engine at normal operating conditions .... take pics and write the temps. taken ON the pics. Then when you develop cooling problems, simply compare the profiles temps. with the actual at hand temps .... the 'bulge' in increased temp is where the restriction or blockage is located.
hope this helps. Good luck, dont scald yourself. ;-)
