My Dakota had the thermostat changed last year, because it wouldn't get up to proper temps. My Dakota will start to overheat after a few miles of driving. It will overheat for about 5 seconds or so and then the temp will come back down slightly for a half a minute or so. It does this back an forth. The radiator is full and isn't leaking. I can smell that the engine is getting hot. I am baffled as to what might be causing this? Any suggestions?