In my 91 3.0, which is of course totally different animal so take it for what it's worth and take fasteddy's advice first...my RPMS used flop up and down when coasting with foot off the gas. Almost like a switch was going on an off cycling it between 1000 - 2500 rpm. Also idled erratically. After replacing the IAC with no change, it turned out to be a fried ECM for me. Just throwing that out there.

98 Montero, 184k.