Dave, Here is a nice explanation of motorboating and cures for it. Scroll down to the paragraph titled Audio Section Troubles.
When I encountered this problem, I always first check for any defective capacitors that are directly across the B+ line. Those are C-142, C-143, and C-132 in your Radiomarine set. I missed the starting question in this thread, so you may have already done that. Mike, W6MAB