Rpm down and up while revving

Hi, I have a 2001 Celica Gt, the problem the car has
is when the engine is cold, let say early in the
morning, you start the car and revv it up, the rpm
will go down first and than up, e.g 1200 rpm went to
800-900 and jump up immediately to where you
throttle it to. It's really irritating when you have to
drive the car like that, you feel like the car wants to
die from standing still going onto the road, it's
dangerous too. After like 5 minutes, the engine
warms up, the rpm will works normally again, which
is not going down and up, but right from 900-1000
rmp standing still and up. I have cleant the throttle
body and the IAC valve and what I found out was
that the IAC valve seal was broken and coolant was
being sucked into the manifold, you can see a red
layer of cooland coated on the inside of the
manifold, I don't think it's a huge amount of coolant
being sucked into the Combustion Chamber. I also
replaced the broken IAC seal with a new one, there
should be no more coolant leaking into the Intake
Manifold. So the questions I want to ask you are:

1) what might be the problem that causing the
engine working in cold temperature fail to work
normally, rpm goes down and then up when you
revv the engine. Sometimes it almost dies I have to
revv it up a little bit e.g 1500 rpm before jumping on
the road from standing still.

2) what should I do with the Intake Manifold that
coated with creamy coolant on the inside? Will it go
away after few months on its own or should I take it
apart and clean it thoroughly?

Thank you for your time to listen to me and I will
wait for your respond.