Eratic idle

Hi Guys, my 1998 STS 55k miles 4.6L, RHD Cadillac idles upto 2000rpm and wont drop when you come to a stop, (the throttle cables are slack and not stuck) so shifting from drive to reverse is really bad, I need to turn the engine off before I shift, also during this condition the engine malfunction light comes on. This fault only occurs after a run. The following day the malfunction light is out and idle is fine, seems to be affected after 15 or 20 miles. Any thoughts, Cheers