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The bubbles are an absolute positive indicator of a bad cylinder head or cylinder head gasket. Have the head checked and the gasket replaced, it is your problem!
If your cooling fans operate then the sensors are operational. Also if the sensor were bad the vehicle would overheat within 20 minutes from a cold start, not after 2.5 hours.
You would have to check with a Mitsu. dealer on the parts, im sure they can get what you need.