Idle problem

Hi everyone, It's been a while since I've checked in here. I have been having a problem with my '69 GTO. When I start the car it idles just fine, but the longer it runs the higher the idle goes. I went to a local show Saturday night (about 20 min away) and by the time I got home the idle was so high I was afraid to drive it into the garage. I just shut the car off as soon as I got in the driveway & pushed it in the garage. The next day I went out and lowered the idle by the screw on the carb & it was fine. Today I went to take it out to see if I solved the problem & once again when I started it it idled fine, but as I drove it slowly started to idle higher. I did not take it for a long drive, so it didn't got too high. Anyone have any guesses as to what could be causing this, and what I can do to remedy the situation. Thanks ahead of time for any help.