High Idle issue?

So I have had my 1992 F150 for about 3 years now. It had one previous owner before
me and it must have been an older man because it was obvious he took great care of
it. Now me on the other hand, I know NOTHING about cars/trucks/mechanics. So I
have tried to take as best care of it as I can with the help of my father and step father.
I have repaired many parts on this truck as its now 22 years old so of course some
things are going to go out. I've gotten very lucky so far in all the repairs I've had to
make, nothing major really and most were easy to figure out. Except the issue I am
currently having.

Bear with me while I try to explain my problem. Keep in mind I'm a woman with no
mechanical experience or knowledge. My truck seems to be running on a slightly
higher idle than it should and when I am stopped at a red light for example, my truck
tries to move forward when I have my foot on the brake. So I have to constantly press
harder on the brake while stopped to prevent my truck for moving forward any further.
When I'm in park, it has a high idle but it goes in and out if that makes any sense?
Like it'll rev up then go back down and act like it's going to die and it actually has
several times. Once while driving.

My step dad suggested the idle control motor which we just replaced and it did not fix
the problem. He also thought that a vacuum seal in the brake cylinder was sucking air
or whatever back in and causing it so he fixed that problem but it still hasn't fixed it.
Any ideas or tips?

(PS So sorry that I cannot explain better!)

Other information: Truck engine is 5.0, 2 wheel drive, extended cab, it has also been
overheating recently but I just assume a hose is leaking because there's no water a
day after I put some in. The overheating started quite a bit after it started this idle
nonsense but I thought I'd mention it in case the 2 have a connection.