1999 Mitsubishi Montero over heating

Did you try flushing the cooling system? Are there any other issues associated with this problem? Is the engine running properly? Any excessive white smoke from the exhaust? Is the check engine light coming on?

Interestingly, I worked on one of these this Saturday. Customer stated overheating and shaking. Identified two bad coil packs, replaced, and it was fine.

Let me know as much as you can.

Okay, the first thing I need you to do is this. Since the check engine light is on, it needs taken to have the trouble codes read. Most parts stores will do it for free. Have that done first. Next, check the battery terminals to make sure they are clean and tight. It concerns me that the lights brighten when you hit the gas. There could be a poor connection, a weak battery, a bad alternator, regulator. And yes, anytime there is an electrical issue, it can cause all kinds of things to happen. Are you certain that the engine is actually overheating? It could be a false signal due to something electrical like overcharging?

While you are at the parts store having the computer scanned, have them load test the battery and check the charging system. I want to know if there is a problem there too.

Sorry, I read it wrong. When it indicates overheating, does the coolant boil over or can you hear it boiling in the engine?

Let me know. It could be a bad sensor.