I know you said you had the battery tested... but is it the original battery? Batteries can be going bad and still test good; all depends on the time of machine used to test them.

Assuming the battery is good, i'd check all of your electrical connections. Make sure the battery terminals are all tight and corrosion free. Check the engine and body grounds.

Check the fuse for the radio. The contacts could be worn a bit.... throw a new fuse in or swap it with an equivalent one (swap it with something minor like the fog lights, door locks, etc)