Tube amp + speakers: after a while, sound begins to cut out regularly, Was: Sound blips (TOS #6) Options

[MrCogline]

Hey guys iv'e got my tube amp and everything set up but i have this problem!

There is a weird sound blip that goes on my speakers it's like the sound stops for half a second then resumes and does this again 3 secs later on both my speakers so it's like pulsing between the two, is it the tubes becomes it doesn't do this when you first turn on the amp.

[MrCogline]

Hi hi,
need some help on identifying a problem i may or may not have with my tube amp and preamp.

The speakers cut out for a second and spring back into life until this happens again 3 seconds later. This happens in both speakers at different times. It doesn't start immediately after turning my tubes on but a minute later . The soft pad where the sound comes from will go in and out as if it's playing a really really low bass note one speakers usually starts before the other. Even if i cut off the signal from the pre amp this still happens. Making me think the problem is in the tube amp.
What could cause this?
My thinking is maybe just some bad tubes, possibly a discharging capacitor?
Possible thermal problem? but i would think that a break in the circuit would stop all sound all together but sound comes out from one channel at all times.

any thoughts on how i could identify this problem?
I have multimeter and probes ready but no tubes i know "work"

The tubes I've got in at the moment are real old.. my dad said one of the power tubes blew and he hasn't used it since.

I've opened the back up and nothing jumps out at me that it's broken maybe i should have the back open and turn it on?

The tube amp i have is the Unisis with the prophet pre amp both from Tube Technology and the speakers are Sonus faber Electra speakers. Here are some pictures.

http://imageshack.us/g/266/pc200048i.jpg/

Tell me if link doesn't work!

Thanks,
Jake.