I got code PO128 about a month ago(coolant thermostat below thermostat regulating temp). I cleared it to see if it would come back and it has not. On a 400 mile roadtrip this weekend I noticed the temp gauge fluxuating from the 12 O'clock position back down to almost the first mark when I would come down a long mountain pass. Looks to me like the thermostat is stuck open. It never overheated and is running fine and the SES light has not come back on. Just looking for some input before I order a new thermostat. Will do the waterpump while I'm in there too. Anything else I should address while the fan and shroud are out? Belts are new.