Oops, replied to a co-poster, not the original poster. My comments seem to be valid in either case though.
If the coolant flow thru the system is adequate to maintain engine normal temperatures without a T'stat, then insufficient with a T'stat, the only contender to accept the blame must be the Water Pump. That is what is failing to create sufficient flow thru the system.
The additional restriction caused by adding the T'stat should not be sufficient to reduce flow to the extent of causing temperatures to rise to 100C.
Please tell me I am

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.