2007 Ford Focus problems starting

I'm having trouble starting my 2007 ford focus SE if it sits for more than about 8 hours. I initially replaced the battery, thinking it had a dead cell, this was not the problem. If I just go into the grocery store and come back out, it starts fine. I have to jump my car off to start it otherwise. The battery is never dead, and I never have to wait when jumping the car, it starts right away. And I have no problem with the car loosing power or acting funny while I'm driving. I had the alternator tested at an auto parts store, they said it was fine. My father in law also tested it, he also thinks it is fine. There is a clicking sound when I try to start the car. He thinks its the solenoid in the starter. I'm inclined to believe him, but I'm a little hesitant because this has been going on for months and I just feel like if it were the starter it would have been out by now? I have another friend who thinks its the battery connection, due to the intermittent starting. Opinions?