retarded idle

ok my car does the 3000 rpm warmup things sometimes
and sometimes it idles at the proper 750-1000 rpm

but most of the time it idles at 1500-2500. It only does this when the engine is not well warmed up, but does not always do it when the engine is not warmed up. ie, sometimes when the engine is only slightly warm, it idles at 2k, sometimes, in the same conditions, it idles at the proper 750.

This problem has gotten better since I got my RB catback (my only mod) but that could just be because of warmer weather