From what I have read 3 years is EXTREMELY good for a glass Chinese tube. My 35 watt one only lasted 8 months and that was without use. I hope all your other tubes in storage are ok as the epoxy holding the ends on seems to outgas into the tube corrupting the gas quality or lets in the nasty impure outside gasses.
If the laser gained power it sounds like it would be a power supply problem more than anything else.
Check your connections to the tube or see if any wires have rubbed through onto the case causing a bit of arcing.
How do the wires connect to the Tube?
If it has the wires wrapped around them then unwrap and reconnect with good quality aligator clips or use earth BP screw connectors with 2 screw holes in them.
Just a stab in the dark for me but probably worth a look anyway.
Rich.
I was just thinking....If the tubes do indeed suck in a bit of outside gas/air then I might run a little bit of silicone sealant around where the ends are glued on and see if that makes any difference to the longevity of the tube. Probably hard to tell as they seem to all have different lifespans anyway.
just a thought but might be worth trying....
I am not completely useless.......I can always serve as a BAD example.