Temp Gauge Fluctuates, Car runs, then check engine light comes on. What's up?

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Last week my Honda temp gauge started fluctuating when idle, but fine when I drove it. A lot of people recommended radiator flush, which I did. That fixed the problem...for 2.5 days. It start up again, but the car worked fine and didn't overheat. Then yesterday, when I started it, the engine sputtered for about 5 seconds and the check engine light came on. What am I possibly looking at?