Mcd60

I own a 2003 Ford Explorer Sports Trac. I have been experiencing problems with the engine turning over immediately after turning on the ignition. This doesn't happen all the time. It's intermittent. Sometimes I'll turn the ignition on and the engines kicks in immediately. Other times the ignition will continue on for 4 or 5 seconds before the engine kicks in. Or sometimes it doesn't kick in right away and I'll turn off the ignition and restart the ignition and the engine starts right up. Any thoughts as to what the problem is and what will it take to fix it? Frustrated!