My 2000 Ultra did this a couple times and replacing the main fuse/CB solved it. My brother's '07 RK Police did the same thing and it was the ignition switch.
I'm sorry, it was 00copglide who said he replaced the main breaker. It's under the seat in front of the battery. They had a recall in 2003 and replaced them with a 50 amp. I take it you don't have a PC tuner, the early ones had issues.