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I would try running compressed air through the cooling system to check for blockages/restrictions to start and then backflushing the system (but not where it will enter the cylinders). The most common problems with these are corrosion build up or sand ingestion. The sand will usually get clogged in the system and cause problems, sand can be extremely hard to locate and remove as well in some cases, corrosion is usually in the exhaust or head and will restrict flow. With compressed air (limit air as well, you don't want to burst or blow lines off) backflush using air, ie: go from the pisser not the water intake tubes and blow backwards to try and breakdown any restrictions.
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