I had something like this happen to me at Saginaw Michigan: I had been flying for 70 hours in previous 2 weeks without any radio issues. Ground control said that I needed to get my radios checked as they were now getting my bleed into tower. I was positioned next to the FBO's line of hangars and parallel to them at the time. After taxi to the run-up area, my radio check was then completely normal without bleed. So, consider your proximity to metal structures that could distort your transmissions before assuming abnormalities with you radios.
As far as your observation that changing your standby frequency made a difference, using my above scenario, was your aircraft moving and in a different position when you made the digit change?