Tharindu Senadeera

Hi Folks, My car is Toyota Corolla AE100. When A / C turns on engine is idling up normally but it cannot hold the RPM, just after idle up RPM again drops. When in traffic it is a big issue because RPM getting low and low. I have changed an ECU but it shows the same issue. Checked all the sensors related to throttle body area. Checked the A/C compressor also. Then mechanic said that the last thing that can be done is ECU remapping. Are there any other thing to check? Need your idea Thanks