I have a PSR-500 programmed with Win500. Just recently I've noticed a periodic interruption on one of the VHF conventional freqs I usually monitor. When a signal is present, the audio is periodically interrupted as if the scanner was checking a priority channel. The S meter remains at full strength. Priority is not slected for any object in the scan list. Wx priority is off. The freq and setting is 155.910 FM CTCSS 179.9. If I set it to search for the tone, there is no interruption and the scanner finds the correct tone. I scan several other VHF convention FM freqs with the same tone that don't have this problem. My Pro-163 receives this freq with no interruption so I believe the transmitter isn't the problem. I have: reloaded the file from Win500, tried deleting that object then reentering it, reentered the object using a different object ID number, initialized global parameters to factory default, initialized the scanner to factory defaults then reloaded the Win500 file. Nothing works. The simple answer is to set the object for tone search vice CTCSS but I'm curious why CTCSS doesn't seem to work with this object. Ideas?