usually about the 4th/5th song is when the (always) right channel goes about 98% out in stereo mode
When you say 98% out, is that 98% of the time or 98% of the volume?
I suspect a loose contact around the base of the tonearm cable. Could it be that as the tonearm approaches the inner tracks, the tonearm cable is stretched enough to slightly dislodge a connector? Or maybe a wire in the tonearm is broken, but normally remains in contact - until reaching those inner tracks?
Otherwise, it seems that the fault occurs as the amplifier warms up for a few tracks. Try starting playback at track four. What does this do to the onset point?
Simplest test is to switch amplifiers - maybe borrow one from a dealer for a trial listen!