Right Speaker music drops, replaced with hum, when playing from Elac 50H Turntable?

My technician and I cannot figure out what is causing the problem.  Upgraded to Grado Red Phono cartridge. After several rounds of troubleshooting, the right channel continues to drop out leaving a steady hum.  Cartridge was replaced, RCA internal connectors replaced, Tonearm cartridge wires/connections checked and cleaned.  The only thing left are the internal wires in the tone arm?  I recently retired and my turntable hasn't been used for years. It was stored in its original box for years.  Any ideas about what it might be and where we could look?  Thank you for your assistance.     
I had a tone arm with a similar problem. The cartridge connectors read good on a meter.  It turned out that the wire had opened at the cartridge connector under the shrink wrap.  The meter could punch through the bad connection so it read good but the cartridge couldn’t punch through.  I found it by pulling on the wire while connected to the meter.  
Check the switch under the arm. It shorts the signal to stop motor clicks when arm isn’t on record.
Swap the arm's left and right outputs into your phono preamp. If the dropouts and hum don't follow the changed leads, then you know the problem is in your phono stage.
Thank you all for your suggestions!  We have switched the inputs and the problem followed.  We believe it’s not in the phono stage.  I wonder about the tone arm cartridge plug in socket connections and wires inside the tone arm?  Thank you again.  We will investigate your suggestions!