Check your radiator, might be faulty fan/coolant leak. Or your radiator pump might be the issueI have a vehicle that does well in town, 25 minutes into hwy driving and the temp needle starts to climb...to the point of a warning lite coming on telling me to pull over and let it idle. Then, the needle comes down to middle range, back on hwy and 10 minutes later...starts to climb again. Put in a new thermostat a year ago and had the system flushed, new hoses and new coolant...same stuff is happening again. Please help.