You do have it mounted upside down don't you? If not that could very well be your problem. The heat produced from the the tube and rising to the capsule can cause the issues you mentioned. If you mount it upside down heat wil travel away from the capsule not toward it. Besides the heat can damage the capsule permanently Since the sound degrades after it's been on for a while I would guess this is the culprit.
BTW, the voltage should always be set the same as the A/C in your country, never change this unless it's set wrong from the factory. You can damage some very expensive gear with an incorrect setting.