If you find it is only happening when cold, then maybe it is just the wafers in the ignition lock getting a little sticky.
If this is the case I suggest getting a small bottle of aerosol graphite lubricant and hitting all your locks with it.
Don't think oh I've got some wd-40 laying around ill use that instead.. I used to lockmsith and it is the worst thing you can put in to a lock aside from sand really...
See if that helps you out any man! Goodluck