The time involved makes me suspect something heat related. I'd suggest opening up the hood and seeing if you can get access to the throttle body, it will probably look something like this:
The throttle cable is the part leading from the green plastic bit towards the upper right to the brownish quadrant near the middle of the image. Have somebody sit in the car and move the gas pedal to see if you're looking at the right thing. Once you find it look carefully to see if you see anyplace where the cable might hang up. If you do this when the engine is hot and the problem is happening you may see that the cable doesn't respond immediately when the throttle is let up.
The problem could be something that is hanging up the cable, or it could be a build up of gunk inside of the throttle body. I'm kind of leaning towards the latter since that is a part where you might reasonable expect both close clearances and heat to change the fit. If you pull the inlet hose (the black item leading to the lower left in the photo, you may be able to see if there is a build up in the throttle body.
I suppose another possibility might be the throttle position sensor (TPS, if there is one). If it was slow to return after the throttle was released the ECU might continue to provide fuel and the engine might be slow to return to idle. This assumes that the TPS is independent of the throttle shaft and has the option to return at a different rate (or it could be what is hanging up the throttle).
Here's a link to a pretty good video of a fellow walking through the process of cleaning the throttle body of a Ford van.