My '93 dodge is a 3/4 ton w/cummins engine within 3-5 seconds. Recently I noticed the temp gage beginning to fluxuate between normal and the next mark up, then back down within 3-5 seconds. It doesn't overheat. I recently had a new radiator installed as the cores were pretty well shot. The gage still does the same thing as before the radiator was replaced.
Could the problem be a faulty thermostat, gage, or a bad heat sensor? The water pump has been replaced about 5 years ago.