Maybe you covered this in the other threads you mentioned, but I didn't see anything about it here so just thought I'd ask to make sure the simple stuff has been covered. Have you checked your anti-freeze mixture lately? An approximate 50/50 mix is generally recommended, since straight anti-freeze or straight water doesn't have the cooling efficiency of a mix, and it's easily checked using one of those $1 testers you can get at any automotive store.

Just bringing it up so as not to overlook the possible obvious. And not that it should be needed under normal operating conditions, but something like the coolant system additive Water Wetter (I think Royal Purple also makes a similar additive) may be worth consideration until the problem is determined.