Check engine light staying on, temperature gauge going up and down quickly.

Is it still overheating? I need to know if the cause of the overheating was repaired. For this motor if the coolant was rusty and there is a lot of electrolysis, then you could also have a bad water pump. The impeller (the fins that circulate the coolant) get eaten away by the electrolysis and the pump is unable to circulate the coolant fast enough. Hence you have overheating and if it is not overheating the temperature gauge will fluctuate as you have described. If you have never had the water pump replaced I recommend you do that. As for the check engine light staying on. there is a hard fault and that means as soon as the ignition is turned on the engine control module has sensed a fault. If the check engine light comes on after the engine has been running for awhile, then it is a conditional fault. That means under a certain condition the fault appears and it is not present during other conditions, this will take some diagnosing. If the engine runs poorly then there are other issues still. Sometimes people plug the wrong oxygen sensor into the harness connectors so they read the opposite bank than it is supposed to. So you need to answer these questions before we can find all your cures. Good luck