1992 Toyota Corolla Erratic rpm

My car would idle at the start up of 400rpm. However, when I turned on the aircon it would go up to 800rpm until such time when the engine is still cool. However, when the engine becomes hot the rpm would read 600-700rpm. When I turned the aircon off, it would dropped to 400rpm. I just have it tuned up recently but still the problem persit. What would be the source of the erratic rpm. Could it be the vacuum? please advise.

Thanks
Allan