'92 Mitsubishi Diamante - Stalls sometimes when come to a stop

Sat for the last two years with maybe a 1/4 or more tank of gas. Wouldn't start, so just put a new battery in. Put some high octane gas on top of old gas and it would stall when come to a stop. We put Gumout injector cleaner in today. It comes close to stalling when come to a stop but hasn't yet in the couple hours since putting in the cleaner; but it comes close stalling because RPM's get down between 4 and 500 when come to a stop., Auto Zone tested alternator today and that's fine. They said to just keep putting high octane in from now on and do the injector cleaner once a month. But why does it idle so low when coming to a stop? Carburetor adjustment? throttle adjustment? did the 2 yr old gas really just gum it up? is it a computer issue with the idle control? I know it could be a myriad of things. Over two years ago it would idle around 800 or less also but not stall, I just want the RPM's up in idle!