What would cause this interuption in a car stereos reception?

[ScyllaandCharybdis]

Only certain stations come on at certain times of the day or night, when before all came in very clear. A station will suddenly cut out (usually in the middle of a good song) without warning. The only thing is I can't check the connection to the aerial @ the units back because I need a special tool. But it doesn't act like a bad connection, no popping or nothing.