my 95 sierra with a 350 has ample power in the summer but when winter comes it has reduced engine power. It starts fine, idle fine, takes off from a standstill fine. there is just reduced part throttle power. I have changed the entire exhaust with o2 sensor, temp sensor on top of intake, cap, rotor, plugs. I have had the truck for 5 years now and it has always done this in the cold. Today was -20 celsius and it would barely pull itself up the hill. Lean maybe? Will backfire from first drive on cold start up if i push it too hard. Take off easy then ok. Any idea?