Ok Ed
This might be a little strange or even unconventional in thinking but I have had this happen in 3 different airplanes including my current new one (to me).
I will be flying along and one out of 5 flights (cross country over a few hours) my radios do the same thing, or they just go quiet.
I have proven that my 12v usb charger plugs cause this and when I un plug them it all goes back to normal. Then I can plug them back in on the next flight and something resets.
Been through 3 different usb chargers and 3 different airplanes and its all random. But when I have problems or stop hearing ATC I immediately unplug those chargers and life goes back to normal.
Always with Com1 being a Garmin GNS and com 2 being something more classic........ took a few months for the bonanza to do it but it finally did 2 weeks ago...
Cheap USB chargers are almost always switch-mode power supplies and notorious for creating broadband RF noise. Usually a MC34063 inside them or similar.
The best option if you're experiencing it from a phone or device charger is simply to try a different brand of charger. No guarantees it won't be built the same way, and very unlikely that anything in the price range of these chargers will have any proper RF shielding, filtering, or bypassing.
Switching power supplies are just RF noisy and it has to be dealt with properly. It's rarely done in a $5 device from the Dollar Store or direct from China. The bigger name brands may or may not do a better job of it.
There's ways to use a linear voltage regulator and they're much quieter at RF, but at the cost of a LOT of heat generated when stepping down voltage. Big heat sinks and a need to get all that heat transferred to the air.
There's also ways to make a switcher that can have the switching frequency adjusted so at least all the noise it's making doesn't "hit" the RF spectrum you're trying to receive on.
I noticed a large number of certified and uncertified USB panel mount plugs at OSH for sale this year.
None had schematics or information on how they were handling a) stepping down 12/28VDC to 5V for USB, nor b) whether or not they properly handle both of the non-standard high-rate charging schemes (for lack of a better way to differentiate them, Apple's and Samsung's/Everyone Else's) for power hungry USB devices.
Assuming they do, and aren't just the standard 0.5A for USB, they're likely switchers and hopefully the manufacturer of the panel mount has tweaked the switching speed to something that won't blast VHF/UHF and GPS frequencies and properly bypassed the input and output and shielded the snot out of the things. Judging by their size though, no way.
Didn't see any USB-C ones. At least not in the brochures or on display anywhere. And the phones and phablets and tablets that need that are coming and being released now...
The "best" 12-28 VDC to 5 VDC RF quiet power supply would be a linear voltage regulator driving a few beefy transistors with negative feedback resistors on them to balance the load, mounted on big old heat sinks, or even a solid chunk of aluminum, but it would be huge and heavy and would get real warm. A nice space heater, but very RF quiet.
Until someone makes one of those in a decent but huge package, you just have to play around with different cheap switchers and try to quiet them down. And you have to test under different loads, and see what they do across the entire spectrum of VHF (and if you value things like ILS and DME and GPS you'll want to see what they do on UHF and L1 also) which is probably the real "randomness" in your scenario. Received frequency and changing loads (iPad already charged vs iPad needing a big charge at 2.1A, etc.)...
Fun stuff. Kinda. Somewhat of a PITA to make a nice RF-clean USB charger for aircraft use in both "12 Volt" and "28 Volt" systems.
