Could be any one of several things (including the jumper as the other OM said). Need a little more inf IMO.
I agree, the bleeders should have nothing to do with it, why they are turning brown is curious. History, have you blown tubes (yet) ? If so, did you disconnect the bleeders and check them? A Little more info may help:
How did the problem show up? (was good until)
Did you change the antenna or feed method in any way?
Did you try quick keying the relay and see if comes back?
I on one occasion had an RF issue that caused a similar oscillation problem. It was a loose connection on my tuner.
Don't discout a bad tube either, Ive have new ones out of the box short.
Hope this helps a little, let us know how you make out. Not that hard a amp to work on if you take your time.