Pretty sure a 4 cylinder engine has to have an idle up since the AC unit strains the engine so much. My MR2 is doing that low idle thing occasionally with AC on. It is acting just like yours without any belt squealing. I'm pretty sure the throttle positioner or idle up is going bad. Yours could have bearing issues that is restricting the belt from free spinning at a normal rate. Make sure you picked up the correct belt also. It should fit in the groove correctly with full contact of the pulley and not be riding up high on the groove.