1985 Nissan Sentra 1985 nissan sentra stalls

I have a 1985 Nissan Sentra with a 1.6L carbureted 4cyl engine and an automatic transmission. i got the car about 6 months ago from the original owner. It is a very well maintained car and ran fine up until a few weeks ago when it started getting hot out. Now when i drive it, it runs great for about 20 minutes or until it gets to operating temp, and then stalls in the middle of the road. After it stalls i have to wait a few hours and it starts up fine again. i just changed the mechanical fuel pump thinking that was the problem but it still stalls. any help would be greatly appreciated. it's a great little car and would be a shame to junk it over some small problem[img:913b4ef2b6]http://www.2carpros.com/forum/automotive_pictures/555366_1001091844a_1.jpg[/img:913b4ef2b6]
[img:913b4ef2b6]http://www.2carpros.com/forum/automotive_pictures/555366_1001091845_1.jpg[/img:913b4ef2b6]
[img:913b4ef2b6]http://www.2carpros.com/forum/automotive_pictures/555366_1001091843_1.jpg[/img:913b4ef2b6]