1993 Isuzu 1.5TD Engine Problem

I know this is most probably the wrong group to ask, but nobody in the Vauxhall (Opel) group gave an answer.
1993 Corsa 1.5TD GLS, Manual, Isuzu Engine.
My corsa once in a while overheats (boils over) for no apparent reason. I have changed the thermostat, pump, radiator and I have tested the cooling fan on the radiator, which kicks in at the right temperature. I noticed in the Haynes manual they mention there is an oil cooler and a oil cooler thermostat, can some kind soul let me know the purpose of these items and if they might be the cause of my problem?
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