2003 Buick Regal Trouble Starting

I started to have problems with my car a few weeks ago when it hesitated to start. I figured it was the battery and went to get it tested. They said the battery was fine. Had me restart the car and it started without a problem. Waited a few weeks and the problem comes and goes. However, just a few days ago it come back with a vengence. This time I must hold the key 10 - 15 seconds and accelerate the engine in order to get it to start most of the time. If it fails to start and I attempt to restart it the car starts without any problems. What could possibly be causing this? I have tried taking it to a mechanic but when they had me restart it the car starts up just fine.