Honesty really is the best policy. If you don't come in late that often, an occasional slip-up can be expected... you're only human, right?
Just say you have chronic digestive problems. It can be used for more things than just being late to work. Especially with meetings and whatnot.
Whenever I get a new job I just announce that I have HIV. That pretty much opens up a world of excuses.
Best Excuse I can think of, I am at the crucial ending of an exciting online game battling the mob boss............