Sixty-Five Bias Creep?

I have a Sixty Five that I replaced the tubes in and then biased because it was sounding muddy, but it started to sound muddy again recently so I took the amp out of the chassis and tested the bias voltage and it was lower than I had set it before. Not by much, probably around 0.07 volts lower. When I raised the bias back to 0.5 volts i sounded clear and as beautiful as ever, but after a few times playing it I found it to sound muddy again, and when I went to check the bias it had dropped again. Any clue what would cause this? Maybe a faulty resistor somewhere? Just wondering where to start checking, or if someone has had this problem in the past.

Thanks!