check gauges light & temp gauge

My 04 Jeep Grand Cherokee's check engine light came on 2 weeks ago the same day the motor for my driver side window died. I took the car in and had the window motor replaced & a temp coolant switch replaced. Since then the check engine light came back on & after having the codes read - the verdict is "engine coolant temp circuit low" - the check gauges light then came on the next day and the temp gauge reads anywhere from 100-240 and hits the red line at least 1 time per car trip over the last few days. This is still happening after replacing the thermostat 2 days ago. Im at a loss for what to "fix" next to get this resolved. The car seems to be running fine- but it really freaks me out when the temp gauge goes crazy and my check engine/gauges light comes on. The Jeep has 67000 miles on it - any advice?