My car has a similar issue. It will run fine when driven and its warmed up. But during idle and warm the RPMs drop considerably Its gone as low as 300 and can stall if I dont give it gas. I've adjusted my idle several times but that hasn't really done much at all.
I believe my issues are tied the fact that I have a misfire in cyl 3 thanks to a bad piston ring but I could be wrong.