In your first post, you said it goes between 210 and 160. That's a safe range for temperature, so I don't think there's really a problem. If it drops when you're driving fast and then comes back up at a stop, that's normal.

If it drops and then comes back up while you're still driving at high speed, it's because your thermostat is opening at too high a temp (then closing again when the coolent temp drops). What engine do you have? 210 sounds like a high temp thermostat. I've got the 3.8L V6, and standard for that is a 195.

Remember that the thermostat's temp is when it STARTS to open, not when it's fully open. If you've got one that's too high a temp, you'll see temp spikes. If it's too low a temp (or if you don't use one, always open) then you'll overcool your engine (also a bad idea). Let us know what year/engine your car is and we can help figure out if you've got the right thermo.

 

I have a 99 Camaro ss 5.7 m6

 

First of all, I agree with nnewquist that you are still running in a safe temperature range. However, if you don’t want the temp to drop down to 160 and you have already tried what the previous posts suggested; make sure you have a stock thermostat, coolant temp sensor, and a temp gauge sender that are all working properly. If you are still having problems, you may want to consider getting a programming tool to reprogram the way your fans work.

I used to have fan issues with my '94 Z28. I bought tuning software (cheaper than Power Programmers and with more capability) and saw that the fan turn-on/turn-off temperatures were way off. In fact, one of the fan's turn on temps had two decimal points in it. I don't know how they got that way, but I was able to re-program my high/low speed fan turn-on/off temps. Now, my car's temp stays in the middle of the gauge with little variation.

You can reprogram your computer to control the fans the way you want if you buy a programming tool. Even if your coolant temp sensor is way off, you can program to compensate. For example if your sensor is reading 20 degrees too hot, you can shift the fan turn on/off temps by 20 degrees. Also, if your fans are staying on too long and bringing your temp too low (i.e. 160), you can program them to turn off sooner.

 

Stock thermostat temp for a 99 SS 5.7L engine is 186 degrees. There's your problem. You can get one for $5.71 plus shipping from www.rockauto.com.

Unless you've been messing with your computer, I think that's probably NOT the problem, but you can scan it if you know someone who has the tool. A dealer can flash it for you for a cost, but I'd go the cheap route first. If it's happening at high speeds, it's not the fans that are the problem. When you drive at a high speed, it's your actual speed that forces air past the rad, not the fans. Replace the thermostat (probably need a new gasket, too), and tell us what happens.