I would have thought it had something to do with grounding. If I'm not mistaken, the signal path should be separate and completely isolated from the rest of the TT, with the possible exception of the ground.
That said, if you're using wire nuts, then there are parts of the signal wire that are not shielded. Maybe that's doing something wonky and disrupting the ground at the table? Even if you get the speed problem worked out, I think you may be subject to a degraded signal. As others have suggested, I'd replace the entire cable. In fact, when the cable on my Sony PSX-55 failed, I installed a set of RCA jacks on the back of the table, so I could use any good pair of RCA cables back to the preamp.
Also, if the signal cable failed, then there is a chance that the ground cable has failed as well, or it could have a bad connection at either end. I'd stick an ohm meter on both ends to make sure it's got continuity, and maybe disconnect and clean the connections at both ends as well.
And finally, try reversing the polarity of the power plug at the receptacle. Maybe that is out of phase with the receiver/preamp?
Curious to know what you find to be the actual problem here.
bs