Need help with my Shifter/Clutch, Pops out of fifth gear when under throttle. Options

[brokenmoped]

Recently I've been having trouble with my shifter, or perhaps my clutch. Here's what happens: while I'm driving on the freeway (the only time I'm in fifth gear for long periods of time), cruising at about 70mph (about 3500rpms) the shifter pops out of fifth and into neutral. I've also noticed that when I'm hard on the throttle, the shifter itself physically moves a few centimeters towards neutral, sometimes popping out of gear. It doesn't do this for any other gears and I'm certain the clutch has been replaced ~20k miles ago. Here's another thing...I have a Weltmeister short shift kit that the PO installed.

Up until recently (last 3 days or so) shifting has been fine and without any problems whatsoever (I make it a point to never misshift). So am I looking at a slipping clutch or is it my shifter that is on the fritz?

Thanks,

Corey