That was what made me think of something else as well Wavrider. I know cleaning the all the switches and making sure all corrosion is off as well as to pop the lid and have a look as well. Like you said, it's just hard to believe it happened to 2 different radios. I know they are old, but it still makes me wonder if there is an underlying issue. I totally agree that all the switches should be cleaned before you rip anything apart and also try plugging in coax to radio and then wiggling each end at both the radio and the antenna. Also you may have some watt instrusion under your antenna or in between the base and SO239. I would check under the antenna first, and if it has a rubber boot remove it as well and check for moisture. This has happened to me before as well and it can make all sorts of things happen. Water is a good conductor of electricity. Just some food for thought. Don't know if you have just installed the mag mount or if it's been there in the same spot for a while. Hope this might help. Again, here's to hoping you find the issue!!