06 tucson V6 why are RPMs dropping

anyone here have or had this issue? sitting at a lite and all of a sudden RPMs drop down to almost stalling out?? happened last year, thought it had to do with running the AC, it was summer, but then all fall and winter it stopped doing it....now its hot again and whether AC is on or OFF its acting up again....feels like it is idling very low and almost stall out, then other things act up when that happens...like trans will slightly buck when i give it gas or needle goes haywire, i can just feel when it all happens as opposed to when its all fine......there is NO ck engine lite coming on, but my mechanic cked for codes anyways and it came up O2, oxygen.....wonder if the sensors are bad or maybe a vacuum leak?? last yr he said about the IAC, he cleaned it up and all and it seemed fine......but now, its really acting up worse.....any suggestions or feedback?
thanks
nancy:uhoh: