Check the SENSOR MANIF TEMP UNDER PLENUM (I think its called) or maybe - the SENSOR CLNT TEMP - its located under the throttle body. If that is detached, which sometimes happens when people work on your car, it will run but not run right. Get in there and look at it from the dirvers side because its next to impossible to de from anywhere else. Look hard its either attached or not, it should not be missing.
Use the correct green coolant. Is this the original radiator? 1985?! Really consider replacing. Check out the thermostat and be sure the water pump is working. That failure can become catastrophic.

 

I blew out one of the top hoses and had to replace it 75 miles from home. Once I got mine replaced and refilled with coolant, I was overheating! The guys here pointed out to me that you have a bleeder valve to get the excess air out of the system, and if that isn't done, it will cause overheating. Just thought I'd throw that in for you.

 

There's no need for anything other than the factory setup which for idle/low speed on the '89 was 226 degrees for the Main Fan and Aux - which should rarely come on - 228 degrees. OR use the a/c which will keep it at least 8 to 10 degrees cooler. At cruise, it should be at the thermostat.

If you took it to a Dealer (assuming they're reputable) and they couldn't find anything, I'd guess they hooked up a Scanner to see the Coolant Temp Sensor Signal and didn't bother with whatever the Display showed. Assuming the Coolant Temp Sensor was steady and within specs, you've got a Display problem. Do your own scan to compare the two signals and to see when the Main Fan cranks on relative to the Coolant Temp Sensor and not your Display.

 

Definitely had similar probs on my 90 but I drive in az 105 degree sunny heat sometimes. Anything above around 90 degrees is too hot for what these vettes were made to drive in according to the owner's manual(And mine of course is black which only adds to the prob). These vettes run hot for specific reasons, but if you're getting up to temps like 260, it sounds like somethings really jacked. Some suggestions you can do are moderate mods and relatively easy yet practical things I did that should be relatively cheap:


First, clean out any materials around the radiator. You'd be surprised how much s*&^ can get stuck up there and fast, which could heat you up relatively easily. Second, swap out that T-stat for a 180. I wouldn't use the factory stat for these cars in hot environments. Avoid the 160 unless you have specific intake mods for it. Third, do a tb-bypass. You can buy the kit I believe for an 89. I think its similar on yours. The tb bypass mod is a relatively simple one just bypassing the coolant from passing through your intake with an extra hose that goes around it. Fourth, you can use a product I've been using in my coolant from redline that supposedly keeps your cooling system about 20 percent cooler. And fifth, you can get an auto switch for your fans after a simple wire-splice install and optional drill for the switch. They should still sell them at aftermarket dealers. I've had mine rigged to always-on for the dual fans under the hood when the ignition keys turned to first position, but theres a switch with it so you can turn it off whenever you want. Also, since you live on a dirt road, it would be a good idea to swap out your crap air filter with a K and N Open Lid like I did. I live in very dusty conditions too, though not on a dirt road and your air filter will have an impact I'm sure without enough cool air coming in as to heating up your intake. The K and Ns as long as they're oiled well are designed to work with the extra dust. These are all not super cheap, but relatively cheap mods that can massively help your cooling probs. Especially if the mechanics are diagnosing your other parts and they're checking fine.

 

When you disconnect the radiator make sure you use a flare nut wrench on the line that runs to your transmission. Maybe some kind of spray may help loosen it too if it's like mine but make sure it's safe to use! Otherwise you may end up with a stripped nut...ask me how I know. I pulled 2 ziploc bags, leaves, and what I believe were some fiberglass strands out of the gap around the radiator. I still need to replace the hoses. At least one swells, which like Oprah, is a sign it could blow! Don't know how hard it is to replace these hoses on a 90' but I will let you know after this weekend.