Car is fine if it sits for long periods but if I.

Car is fine if it sits for long periods but if I try and make a few stops on my way home from work such as the grocery store for 15-20 minutes, come out and car won't start and have to let it sit for 3 hours. It has done this off and on for sometime. Use to only have to wait 1 hour, then went to 3. Got a new key fob. Thought it might have something to do with anti theft device as that is what triple A guy suggested. The anti theft light is flashing and car won't turn on. Took it to dealership and they replaced something in steering mechanism that had to do with anti theft device but I think they only did this cause that is what I suggested was problem. Worked for a week then wouldn't start for an entire day. Took back in and they can't figure it out. They have had my car for over 2 weeks. HELP