Engine sputters and stalls? Code P0171

I have a 1999 ford ranger 4.0 v6. I tried to get it smogged and it said code P0171 I took it into the shop and they said its a lean code, but they couldn't see what was wrong they said they tested everything but couldn't find anything. But now my truck sputters going down the road its really stubborn and doesn't want to shift and even dies sometimes then has a hard time restarting when it does die, it seems to happen more so when its at normal running temp. Any solutions? I've replaced the maf sensor egr valve throttle position crankshaft position sensor