Probable answer
This summer I was having the exact symptoms on my 90SC. 115+ temps here in Arizona at the time. Around town no problem. Climb a hill and it would immediately heat up and within 5 min would be past the "m". If I didn't stop within another 2 min it would trigger the check guage light. Now, this was a new problem. I did all the diagnostic, fan working, etc. etc. Put on two GM air dams all to no avail. I worried about hg's but didn't have any of those symtoms. My radiator visually looked to be in great condition, as I've always religiously changed coolant. However, I replaced the radiator with a new copper/brass one from Radiator Express (price not much more than having your radiator taken apart and rodded). Problem solved!! I could climb any hill as fast as I wanted to go and the temp stayed about "r" on the hottest of days. Bottom line is I think I clogged it up myself by using water out of the garden hose. High alkali content around here. So I've learned my lesson, no more hose water, only distilled for me from now on. John