Hi all. My brother's car (Gen-2 1.6 MT year 2004) got overheated because of thermostat failure. After he changed the thermostat(original proton) the engine goes well for about 2 months then it got overheated again also because of thermostat failure. He changed the thermostat(ori proton) and do the top overhaul but the engine got overheated during the test by the mechanic. The mechanic expected the engine head got warped. He said usually Campro head cannot be skimmed but he knows a machine shop who can skim the Campro head perfectly and he claimed he has done 3 similar cases before without any further issues. After the engine got skimmed, the mechanic also cleans the radiator and change the water pump. After the engine head been reinstalled the mechanic found out the RPM got crazy(went up and down crazily). He went for compression ratio check and did the retuning but still got the RPM problem. Later he took out the thermostat and the RPM problem settled. Later got new problem, the Check Engine light will start to turn on once the the engine reaching 3500rpm. My brother went back to the mechanic for few times but still no change. He also tried to bring his car to other mechanics but still got the problem. The temperature is fine, never exceeding 90 degree celsius, but during driving it went lower than usual. But my brother cannot drive his car calmly as the Check Engine light still turned on once the RPM went 3500. Any opinion sifus?