What I mean, is maybe the engine is hotter than you think, the gauge may be off. I don't think 100C is normal. I'd check that you have the right mix of antifreeze and then replace the thermostat.

PS: How are the fan/water pump/ps belts?

I think 266K on the timing parts is really pushing it. At the very least, I'd change the tensioner and (at least) the upper guides. The spring in my tensioner was noticeably weaker than a new one at 85K. A worn chain will affect timing.

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