99 Idling Problem

Hello all,
I've got a '99 GT 5-speed that I bought in 2001. I've pretty much
only done cosmetic work to it, and have had no problems with it
mechanically.
Two weeks ago, it was very very cold (below freezing) for a couple
of days. I went to start it up, and it turned over, but the idle went
straight down and the car stalled. I got the car running by giving it
gas right when it started, and it was rough for a second, but then I
easily held the idle manually via the gas pedal like normal. Once I let
my car off the pedal, the car stalled.
I went back the next day with some dry gas, and manually idled the
car again for 10 minutes. The problem still persisted. I decided to just
drive it and open it up. I couldn't even get the car into 1st gear after
going into reverse. However, when I started the car, it worked normally
all of a sudden.

I was wondering if anyone had any ideas,

Thanks.

check the iac valve, search for it online, but it should be the black
valve looking thing on the throttle body facing forward.

the iac valve, or idle air control valve, does what it is called, and
controls the amount of air into the engine at idle. my auto 99 had this
idling problem as well, and all you need to do is to get a can of carb
cleaner, and take the iac valve off, and spray inside the iac valve to
remove any carbon deposits, because the carbon deposits are holding the
valve portion closed, thus not allowing your mustang to idle. sometimes
it releases, as you said when it worked "normally all of a sudden"...