A word of warning, some of the newer Airport Expresses have brief but regular dropouts through airfoil. I'm not sure if this is always the case but it was with my one.
Airfoil claim it is an Apple issue of course, but who knows?
Luckily, you can try airfoil free for ten minutes to test if this is the case (reinstall for more testing).
I'd be interested to know if anyone else had this problem with airfoil > AE ?