1991 GMC Sierra starts, then stalls when hot

[57chevyman]

hey ya'll,

my brother has a 1991 GMC Sierra... it has the 5.7 engine.

it starts and runs great when cold. idles nice and smooth.

when the engine is hot and has sat for about an hour after shutting off, it will start and stall and be very hard to start. the problem only happens on hot summer days. if the truck sits long enough to cool off, it starts and runs normally.