[fenbach]

keefer,
of course, the simple answer is - just remove the buffer cap. but, according to the AAR website, "Whether you use a Solid State or mechanical vibrator, both the radio manufacturer and the vibrator manufacturer recommend that you replace the buffer condenser each time you replace the vibrator." implying that the radio should have one even with a SS vibrator.
i have to agree with chas. that something is loose or touching. check the voltage at the rectifier tube with the buffer cap in place and with the radio oriented both ways. to see if you have high voltage when the radio doesn't play. also, try removing the AAR vibrator and connecting it on the bottom side of the radio with wires and alligator clips. that way you can rotate the radio and the vibrator separately and at least determine if the trouble is in the radio or in the vibrator.
quite the mystery. hope you're still having fun!