I have been using some brand new BK CRT 30MHz scopes for a few years now and they do the same thing. The problem is not that the oscilloscope is old or wearing out, it's that the speed at which the electron beam is moving across the phospherus screen has to go up with the frequency. It needs to be at leas twice as fast as the frequency you have it set to. The reason it's dimming is because it's moving so fast that with the normal human eye you can't see the extreemly dim, quickly moving light. The simple fix is to turn up the intensity of the trace.