Erractic Temperature Readout On Dash

I have noticed lately that my temperature readout on the dash seems to be a bit erractic on my 93 S-10 4.3 Blazer. Sometimes it seems to be too high and sometimes it seems too low. First thought is that the thermostat is bad-right...well that doesnt seem to be the case as I can let it set (not running) for 1.5 hours in 50 degree ambient and it is still 220 degrees when I start it up. I pulled the plug off the sender on the passenger side head, it went to no reading on the dash and then put it back on- not really any change from before. What is the history here???- bad sender or bad gauge??? Also, does anyone have a link to a web page that descibes how the CPI system works and all its sensors?