95 PA codes.

Let me tell you the problem that I'm having. When I get in and begin to drive the car it stumbles for a couple of minutes. This only happens after the car sits for hours, and only for the first few minutes of driving.
I took it to the parts store and they ran a diagnostic for me.

History Code P0171
Fuel Trim
Sensor Signal Lean

History Code P0300
Misfire Signal
Misfire Detected

I took the car to the shop, the guy drove it for thirty minutes, came back gave me my keys back and told me to go and get some complete fuel cleaner. I did that and had the fuel filter changed, he told me to go through two gas tanks full of the high octane fuel with cleaner. Can anyone tell me what these codes mean and a possible solution?