I have a 1990 Honda Accord with approx 206,000 miles, car runs great but when car warms up about a little under the half way mark on the temp. guage my " S " light starts to blink and RPM's increase

I have a 1990 Honda Accord with approx. 206,000 miles on it, car runs great the only problem is when the car warms up a little under the half way mark on the temp gauge, my " S " light starts to blink intermittently no loss in power but you can feel a little kick from the tranny and the RPM's kick up a little like 3000 to 5000 rpms higher, but like I said only when car warms up does this happen on cold nights the light stays off, and would this problem cause my vehicle to emit higher NOX emissions if " S " light is on or is that a separate issue all together,