Erratic, rough idle only when warm, sets: p0171

Question

Rough idle, only @ op temp. Comes & goes. Idle drops 150-200 rpm, shudders. Sets code p0171: w/ increasing frequency: now every 20 mi. Or less. Replace m a f, I m r c (both), I m gaskets (upper & lower) & bolts, air filter, plugs, wires. Cannot find any vac leaks @ pcv, power brake hose or I'm gaskets. E g r & d p f e seem ok. Only problem I found: I a t sensor resistance increased when warmed up. Specs said it should decrease. Could this contribute to rough idle or code? HELP!