What would cause rpms to drop off and vehicle to slow down when engine is cold?

What is your question?

When I start my pickup and take off, it starts slowing down and will stop eventually. It makes some popping noise and eventually takes off. When you let off the accelerator it runs normally. I have replaced the cap and rotor and spark plugs. Any ideas what else it could be? Had it scanned and O2 sensor came up, but nothing else. Mechanic thinks something else it causing this code to come up and that that is not what is causing loss of power when engine is cold.