how is your radio reception when you manually tune it? Typically it will only do that if it can't find a signal strong enough to lock on...
check if the antenna is screwed down tight, make sure there aren't any broken wired from the radio to the antenna.
When was the last time you actually had to use the scan on it? I know personally I set my radio once then never touch it again unless I disconnect the battery...also, are you only trying this in the garage or something like that? If the car hasn't moved while you have been trying this, maybe the spot your at has bad reception.
If none of this pans out, you may just be looking at a radio going out...the guy doing the warranty work on my Stang says he sees lots of them come back with wacky stereo problems.
SOLD: 2003 White GT
flowmasters, UPR Blue Thunder shifter, UPR O/R X-pipe, 4:30 gears, tuned by modularpowerhouse