Replaced battery,tuneup, replaced all filters/fluids/plugs & crank/can Sensors.

Mechanic checked fuel pump pressure and the voltages. No luck. During springtime and fall, I can let the car sit for over 2 weeks and it fires up with no problem. Also, the car has no problem restarting or running during the day. My mechanic just gave me a gas additive.....the car seems to start better, but, so far, I have only used a little less than a half tank with the added gas additive.