Help! Overheating prob with daily driver

[90'5.0 Conv]

Alright I know this isn't the Honda message board but I thought maybe I could get some help here. If this is the wrong spot for this, I apologize. I know when it comes down to cars though, cars are cars and maybe someone can relate to my problem. Today, I was driving my 91 honda civic home from work and it started to overheat. It went up and down for a bit(ten minutes), then finally went sky high, luckily I made it to the house before it got too hot. Anyway, the car has 213,000 miles on it and I just changed the plugs a few weeks ago and unfortunately the #3 plug was covered in oil. So, I realize I have a short life to live on the motor but I'll ride it out. Like I said, it started acting up today and I've checked out a few things. First thing I did was change the thermostat, that didn't work so i checked and made sure the fan was coming on. The weird thing is when I'm idling the car or just leave it in park, i can rev the motor and everything is great till I head down the road, then the problems start. Everytime I head somewhere, it gets hot prettty quickly. I was thinking maybe the transaxle was giving me a problem. Is that a possibility? Why would it just overheat when I'm driving yet when I rev the heck out of it in park, nothing? Thanks a bunch for any advice. I hate to drive the mustang daily, gotta take care of my baby. I'm sure you understand Anyway, all ideas are welcome and again I'm sorry if this is inappropriate but I have nowhere else to turn to for help.

Thanks, James