Please help with windstar problem - P1518 IMRC code

I've been having a problem with my windstar revving badly for a while now. Especially when the heater or A/C is on, the rev's go down to about 3-500 then back up, then back down... Sometimes the battery light will come on, about to stall, then it goes back up. It does this even when the heater off, but its definately worse when it's on.

My engine light is on now as well... we got the code "P1518 IMRC stuck open".
My husband is pretty handy with vehicles but I'm trying to get as much info as possible, so he can fix it himself as opposed to spending a lot at a mechanic.

Could anyone shed some light as to whats wrong, what part we need and how he can fix it?
Thanks very much for your time!!

It's a 1999... sorry.. forgot to mention that.

I'm guessing yours is a '98 or earlier?

I'm guessing yours is a '98 or earlier?
Her member profile show a 1999 model.

They switched from vaccum driven actuators to electrically driven in '99.
I seem to recall posts about nylon clips breaking on the '99 and newer as being an issue.