What Almarg said.
With reference to Zd's idea, I cannot imagine how "knocking" the cartridge out of alignment could produce the phenomenon that you described, but you're going to find out whether the cartridge is the cause by swapping channels all the way up the chain.
Unfortunately, IME, when one has an intermittent problem such as this it typically fails to reveal itself when one is hunting for it. I have been trying to find out for 8 months what is wrong with my Sony CDP. It malfunctions in my house but not on Bill Thalmann's workbench, except for a brief moment which allowed us to confirm that I am not imagining things.
With reference to Zd's idea, I cannot imagine how "knocking" the cartridge out of alignment could produce the phenomenon that you described, but you're going to find out whether the cartridge is the cause by swapping channels all the way up the chain.
Unfortunately, IME, when one has an intermittent problem such as this it typically fails to reveal itself when one is hunting for it. I have been trying to find out for 8 months what is wrong with my Sony CDP. It malfunctions in my house but not on Bill Thalmann's workbench, except for a brief moment which allowed us to confirm that I am not imagining things.