Yaqin MS-300C Integrated Amp Keeps Shutting Down

Hi, I've been stumped for a while with my Yaqin amp shutting down. Most times when I turn the unit on, I have no output until I touch one probe of my multimeter in test opening, then it comes on. Many times however, I can't even get to adjust the bias for 24 hours as it appears to go into protective mode. After the 24 hour wait, the protection circuit goes off and I can bias the 300B tubes to .8 Volts per the manual. However, after a day or two, the process repeats itself: I have to touch the probe to the test point to start it, or it shuts down for 24 hours. I bought it used, and was told there were few hours on the stock tubes. Could it be the 300B tubes are just not holding their bias? I've used the 4 ohm and 8 ohm taps with several speakers with the same results.
Any help would be greatly appreciated.