I have a 91 ranger with a 4.0 L v6 engine. I have done repairs on her(replace head gaskets, replace slave cylinder, clutch, etc.) but now it has a VERY unusual condition. Whenever I cold start, it takes a few seconds, but it starts. But when I go to the store and shut the engine off and go to turn it on again only 10-15 minutes later, it will start and reach idle rpm, but then immediately die every time.

This is where it gets weird:
This condition only happens one a week, and when it does(I have learned from doing this multiple times), all I have to do is wait one hour and then it will start up fine.

Any and all help would be very appreciated. I am thinking it has to do with the air-fuel ratio, but what do I know? Thanks to all who read this in advance!