Let's go back to my theory -- sounds like you're having connection problems at the pins/socket of the tubes.  If you are, that will kill tubes.  So that's what I think your problem is, as I've said before. 
 
Hirsch uses a silver coating to make sure each pin gets a good solid contact with its corresponding socket, you may want to try something like that (but be careful not to create a connection between adjacent pins, etc.).