97 Chevy S-10 p/u gives me lean fuel when warmed up

Ok, here it goes.. A little backround first: fuel pump is a yr old, changed out fuel pressure regulator, fuel filter, and all spark plugs, fixed vacuum leak, fuel pressure gauge reads 55 psi when running. When I first start it up it runs fine when its cold, not a sign of problems. After about ten minutes it starts to act as if its not getting fuel, step on gas and nothing happens. It wants to die. Check engine comes on and reads p071, running lean on fuel. When its having trouble fuel pressure will drop when I step on the gas but after I let off gas it goes right back up to good pressure. That tells me fuel pump is good. Where I get confused is why does it run fine when cold and it takes a while to start having trouble. What would cause this? I'm 600 dollars deep and still have no solution. Please help! Thank you in advance