That happens when the voltage drops below the minimum voltage for the radio when batteries are under heavy load. I suggest you connect a Volt Meter across the two batteries and check it when you are experiencing the problem. More than likely that is the problem. Check water level in all the batteries and all battery connections. How old are your batteries? They could need replacement.
Those two batteries will discharge more than the other others and then when you do a regular charge they may not completely charge. If you use that configuration you should alternate the batteries used once in a while or they will get worse and worse. If you do change to another two, I would remove the two you have been using and charge them connected together (in series) with a 12 V charger just to make sure they are fully charged and then re-install.
A radio alone shouldn't pull more than 8 to 9 amperes current (max), if you are using the same batteries for an amplifier or other accessorories (lights, etc.) you should definitely install a 36V to 12V converter (spreading the load across all batteries), capable of supplying more current than your total accesory current draw OR, as some recommend, add a separate 12V battery just for the radio (would require a separate 12V charger). Hope this helps.
Any other questions, please don't hesitate asking.