This is an extremely common problem with PA50s. Put your air filter back in!
I think it is caused by the transmission running out of "gearing" while the engine still has power left to burn (like pedaling a bicycle at high speed in a low gear).
<font size="+2">Option 1: Variator modification</font>
You can <a href="http://www.mopedarmy.com/forums/discuss/1/7890/7890/">modify your variator</a> to let it "gear" further past spec, or you can try tightening your belt.
<font size="+2">Option 2: Belt tension</font>
I find getting the tension right on the belt makes a world of difference on my PA50II. This is a lot easier if someone helps you.
1. The rear wheel is held on by four bolts, <i>loosen</i> them, but do not remove them. The top two bolts act as a hinge, the bottom two bolts lock the rear wheel in its adjusted position. Loosen the chain tensioner bolt as well.
2. Tighten the left (belt) side first. Consult the attached diagram for information on how tight the belt <i>should</i> be. I find that moving a little away from spec (tighter or looser) will give me better (or worse) performance. You'll have to spend an hour experimenting. While you hold the tension, have your helper tighten the bottom-left bolt.
3. You'll notice there are little graduation marks on the frame at the bolt you just tightened (you might have to brush away crud to see them). Use them to tighten down the right-bottom bolt at the same spot you tightened the left one so your wheel will be straight.
4. Tighten everything up. Don't forget to tighten your chain tensioner as well.
Your belt should <b>not</b> slip! If it does, it is either too loose, or you have oil on your pulleys (clean them).