2000 GMC Sierra Won't start in cold wx and runs rough in pa

Usually will not start if less than 50 deg F. When I punch it to passing gear it feels like its running on 2 cylinders.
Dealer had it for 1 1/2 weeks. He THINKS! It's the fuel pump he said it has 50lbs of pressure but should have 60lbs. I didn't want to spend $800 to find out this wasn't the problem. It has been tuned up with new wires, acdelco plugs, distributor cap and fuel filter. Same problem. I guess I was kind of shocked to realize they didn't know what the problem was for certain.