Ian, had this problem with my Garmin radio -SL40 (radio breaking squelch after some time and worse on certain frequencies)

you can adjust the squelch in the set up mode (scale 00 to 100 ?)  

I tried this but didn't fix the problem

Possibly should have just replaced it worth an I-com of similar size

Instead sent unit off to California for a holiday at Garmin factory and had work done on it-cost $800 as out of warranty

works fine now

Gerard