I have a 2002 Nissan Sentra. In Nov. 2010 had inspection done and replaced upper radiator hose at that time. Right after that, I had no heat and the temperature gauge would go up to hot but then come right back down. Brought it back to mechanic, changed thermostat and both things ok only part way home. Started riding with coworker only driving to her house 7 miles away and still temperature gauge up and down, stopped driving completely in Feb. Just brought it to dealer who replaced thermostat and worked fine all the way home and then started all over again the next day. Seems to happen faster at higher speed but will eventually do it in any driving conditions. Today service engine soon light came on. Obviously not thermostat problem. Could it be water pump? Dealer did pressure test and that came back fine so no head gasket problem although don't have a lot of faith in that since problem still exists and they thought thermostat would fix it. Don't want to keep spending money on possible problems, only actual ones. Any help would be greatly appreciated.