did you check the antena conector? Those things sometimes corode. Also check the antena mast, most are screwed on and could be loose and/or coroded.
did you try pluging in another antena to test? It could be a bad wire at the antena end. Any GM antena will do for a test.
It could also be the radio. there might be a cracked solder joint or trace in the thing. There are several boards crammed into the radio. it could be a coroded plug in between boards.
Try removing and inserting the bulk connnector on the back of the radio. a bit of corosion there can do odd things.
Did you know you can "crash" these radios just like Win9x does to a PC? Power surges, and some short circuits, can do some rather odd things. I managed to blow the lighter fuse once and the radio would not power on until I finally disconnected the batery to clear it out. I've seen that happen in other GM cars as well for no apparant reason at all.