63 Dart Radio issue

Intermittent problems are always the worst. Could be something "as simple" (but hard to find) as a crack in a PC board trace, or a bad solder connection. Some problems come and go with heat, cold vibration, or a combination.

As mentioned above, make CERTAIN it is getting proper voltage. Make certain it is grounded properly, and that proper voltage is ACTUALLY getting to the radio side of the 2 wire connector. One wire is red (usually) and is power. The other is orange and is the dial lighting.