I've noticed that my coolant expands quite a bit when I drive around, so much that I'm loosing coolant through the expansion bottle. When I park after a lengthy drive, I'll usually hear the coolant bubbling via the overflow bottle.
When just sitting and idling ( without having driven around ), my engine temperature looks likes this:
I can sit and idle like this all day without any problem or any coolant overflow. It's only if I drive around for a while that the coolant starts to expand and overflow the bottle, and as long as I keep it topped off there are no overheating problems.
I changed my thermostat not too many months ago, and my heat seems to work OK so I don't think the heater core is clogged ( although I should probably double check that ).
I recently changed the seal on my oil cooler and when I drained the coolant noticed black gritty sediment, but no trace of oil. While I had the oil cooler off, I flushed it and also saw a ton of this black gritty sediment.
I did compression tests a few months ago, and had very good compression on all cylinders, so I don't think it's a blown gasket unless of course I've blown it in the last thousand kilometers or so.
I've blown three different coolant hoses in the past year, and the car is barely driven. It seems to me that excessive pressure is building in the system when driving, but I'm not sure why.
98 Mazda 626 2L running premixed Prestone green coolant boilover rated to 259*F/126*C and the engine never gets even close to that temp unless I forget to keep it topped off.