OT: 98 olds Aurora troubles, can't figure out cooling problem Options

[aircooledboy]

Since the members of this club are some of the best gearheads on the planet, maybe you can help a brother out.

I have a 98 Aurora with 104,000 with the 4.0 "little Northstar". Several weeks back, it started to have cooling problems. After about 10-15 minutes of driving, the heat would quite suddenly go cold, then the coolant temp would climb to anywhere between 225 to 255ish. I have flushed the coolant, changed the t-stat twice, pulled and inspected the water pump, replaced the coolant cap, and examined all the spark plugs. Nothing. (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

I would assume it was a head problem, but for this: on a couple occasions when I was driving it and the above crap happened, before I could get to where I was going, just as suddenly, the heat would get hot again, and the engine temp would fall back down to 210-225 and stay there for a while, then 5 minutes later, no heat, temp would rise a little again, but only to about 230-ish, stay there and no heat. All this would happen on a trip home from the office, which means I'm moving 95% of the time, anywhere from 40-70mph.

I have driven the car probably 10 times since this started happening, and every time it will do most or all of the above like freaking clock work. With the problem coming and going like it does, it seems like it couldn't be a head problem, but I don't know what the heck else to look at now?

Ideas? (IMG:http://www.914world.com/bbs2/html/emoticons/pray.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)