Car overheating...

I have a 97 Gle that recently started running hot when I drive for more than an hour. Sometimes it will get hot enough that when I shut the car off a small pool of antifreeze will form under the car. So far I have changed my thermostat, and got a new radiator cap. I thought this would fix it, but no such luck. Any suggestions on what my next step should be?

Marlon...

Forgot to mention my fan does come on.

Just went for a drive a few minutes ago. Since the weather is warmer the car gets hot much faster. When I parked my car antifreeze was leaking so I opened hood and noticed the coolant reserve tank was much fuller. Looks like the antifreeze from the radiator is backing up and overflowing the coolant tank.