Why would my idle be high sometimes and then other times it will be normal?

I have a 2002 ford ranger that I ran with a vaccum leak for at leat a year before my husband found it. When he repaired it, he also changed the transmission fluid and filter, and the oil filter and fluid and then cranked it up and ran it for 30 mins and then the idle went sky high to about 3,000 rpms, so he brought it back to the house, and we discovered we had a bad Idle air control valve, so we replaced that, cleaned the throttle body and then disconnected the battery. It drove like a dream for awhile but now sometimes when I come to a stop, it idles up to a higher rpm, and I have to put my foot on the gas to make it drop down. So my husband put dielectric grease on the IAC and connecters, and it didnt do it for about two days after that, but then it started doing it again. I live in louisiana and the humidity is very high here on most days, and it only seems to do it when its humid. Is there something seriously wrong with it, or is it just sticking maybe?