FWIW - I just returned from a trip to Key West and saw a similar situation on the Monroe County PS system which is a Motorola Type II SmartZone system (vs. an X2 or P2 system) although it was digital voice. It looked like the radio was reacting to the CC activity and tuning to the voice frequency but had trouble holding the voice channel.
Sometimes it did this and other times it was fine on the same TG which leads me to believe the radio occasionally locked onto a different (distant) site or something was interfering with a specific frequency on the system at times.
Future PSR-900 user...(still hoping!)
PSR-800 x 2 | PSR-500
BCD-396T | BC-296D (voice) | BC-245XLT (mTrunker) | BC-600XLT | BC-IV | VX-8R
Not a Radio Shack fan.....