My 2006 BMW 330xi's idle seems to 'skip' (or get weak) while waiting/stopped at a stop light (doesn't cut off.) It only happens intermittently. More so once the car has been driven for a couple hours in the same day. It never happens first thing in the morning, but more frequent at night. No indicators on the dash, so BMW can never 'find' the issue. (Approx. 60,000 mi) The car drives fine at high speeds, no concern there. What could be the problem?