If you tested the thermostat in water and it's opening fully at the correct temp, than your problem is almost certainly something else (possibly the head gasket, or water pump from your symptoms).
What temperatures is your engine reaching, and under what conditions? How are you measuring it?
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'84 760 D24T/M46
'86 Isuzu Trooper Turbo Diesel
'01 VW Golf TDI
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