Btax,
If I understand you correctly, you clearly have an intermittent (aka bad connection) that could be just about anywhere. It could be any where along the line.
You changed to another cable and it worked for a while?
Did you check the 50 at the post again?
If any of your cables are not molded end, get out a screw driver and open them up and look for the bad connection.
If you try to diagnose this with a "voltage" detector, that will drive you crazy because there will always be enough AC around to indicate power as there.
Matt