I have a 1996 Blazer, and when I start it up within minutes it shows the temp to be very near 260. After a few minutes of driving it drops back down to the correct temperature. When the temperature reads correct, I have heat. When the temp goes up really high, my heat gets cold. Can anyone help me in diagnosing my problem?

Sounds like the thermostat to me. It's not opening properly, so the coolant isn't flowing through the heater core.... hence, no heat. The engine starts to overheat and it pops open. Bingo! Heat!

I had the same problem years ago in a '91 Checy pickup. I changed the t'stat with a GM one and 6 months later - same problem. I then changed t'stat with a Robrtshaw made t'stat and my problem was solved for years.