1987 Ford Escort Wagon radiator

[introspectiion]

I'm having some issues with the radiator. It started leaking, so I put in some of that sealant stuff, and that seems to have fixed it, as well as the leak in my resevoir. It does okay now, but whenever I'm sitting in traffic....or sitting anywhere instead of driving...it tries to overheat. What could be causing this?
I've also noticed that sometimes when I'm idling, and only when in Park, the rpm's will go up sharply, then drop back down. Or they'll go down, and the car will shudder, then they'll go back up to normal.
Any advice or idea's would be greatly appreciated. Thanks!

[AzTumbleweed]

Sounds like the radiator is clogged up and will need replacing or rodding out.