Good to see you're keeping these old machines working and in circulation, @M1JWR.
I've been using the MDS-JE500 this week and I think it's probably just a case of the older style mechanism not being quite so slick as my MDM7 decks, all of which have had belt replacements fairly recently.
One pleasant consequence of using the JE500 is that I've had to dig out some of my older MDs to listen to as most of my more recent recordings have been done in LP2. It's been nice listening to a few things, especially old "mixtapes", that I hadn't heard in a while.