Mark IV help!

[Miiks]

Hi there. Recently i have found some problems with my IV. After having played a while, the volume suddenly goes very high and i get some noise like blowing in a microphone. The volume thing happens only on R1, but R2 has some unnormal rattling and LD has a quite loud humming. What's the problem? I have changed all my preamp tubes about a year ago. I use 2x6l6 and 2xEL34 in the power section, the EL's are about 2 months old and the 6l6's are original tubes from 2008.
I'd really appreciate if anyone had a clue about what i could do.

 

[mule#1]

Are your 34's in the outer sockets? It sounds like a bad tube to me.

 

[Miiks]

Yes, the 34's are in the outer sockets. I forgot to mention that after switching from R1 to another channel and back to R1, the volume goes normal again, but after a while it gets high again.

Do you think it's a bad power tube?