Ford Transit 2.5 diesel "N" reg

I might be buying a Transit with a problem. Starts Fine runs well, but every so often, totally random times it smokes like a trooper, then stops just as randomly. I think it might be a
temperature sensor, but can anyone here help further. Thanks guys, the guys at local ford ncouldnt have been less helpful, "it could be anything" was all I could get from them.
Thanks again in anticipation
Phil Gavin M.I.M.I.
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