1997 Eldorado

I have a 1997 Cadillac Eldorado with 107,000 miles with the North Star 3.2 motor. The car has run perfectly until the last 2 weeks. It sounds like it is backfiring when I try to start it. It may take 3 trys to start, then it runs fine. This only happens after the car has set overnight or during work. My fiance has replaced the original air filter with a KN filter last year. I had the filter cleaned this past summer. Do I need to clean it again? Could I have gotten some bad gas to cause this? My fiance recently passed away and he was a certified mechanic, so I am lost as to what to do. Thank for any input.