Clutch Problem

I have a Pontiac Grand Am Se 2002, 2.2L DOHC and has about 80K+ on the mileage, this is the first major problem I've experienced with it. The clutch becomes limp, has no resistance at all when the weather drops to around 40 to 50 degrees fahrenheit, I have to pump it several times before the resistance goes back. I noticed that when it's a normal day, about 70 to 80 degrees fahrenheit the clutch is normal and doesn't show any signs of limping. I took it to a Firestone car repair shop and they didn't see anything wrong with it, unfortunately it was a warm day when I went so it wasn't showing signs of limping. I told them my story about the cold weather incident and they just smiled at me and said that it doesn't make any sense. I don't know much about cars so I hope you guys could help me with this one. Thanks and have a nice day.