Your map cd goes into the same slot as a music cd right? Do you get the same problem with music cd's. your GPS system should be a receive only setup (and you're inundated with signals from the GPS satellites all the time anyway) so if there is a conflict it could be from EMI from the cd motor or the electronics connected with it and not from a purposeful signal out of your nav system.
Otherwise maybe the FBI's got you bugged!
That's what I'm here for; sage, yet disturbing advice.