I have a 1999 blazer 4.3L 188,000 miles. After a cold engine startup I drive it a few miles and it heats up to 210 after a few minutes it will rise quickly to 260 and the check gauges light come on. Then after 3-4 minutes of driving it will quickly dive back down to 210- 215 range and stay there. I replaced the thermostat first, no change. Also the antifreeze looked brand new in the housing that holds the themostat. Next I replaced the fan clutch, It was wore out and did not work the way it should, but still no change. My brother was a GM mechanic but he is stumped too. If it were a clog wouldn't it just get hot and stay hot? I am trying to sell this vehicle and don't really want to dump any more money into it, but any ideas?