This is almost always caused by using too much water on the finer grits. What's happening is you are hydroplaning on the disk. As you are grinding, you are actually floating on top of a layer of water, every so often you will displace enough of the water so your piece will "grab" the wheel. You may feel like you're grinding along quite nicely, but it's actually very inefficient.
Simply reduce the amount of water flow that you are using on the wheel and that should fix the problem. The other possible cause is that you are not putting uniform pressure on your glass while grinding.
We've actually also had some customers who start off using too little water. This can have the opposite effect of hydroplaning where you are actually creating too much friction between the diamonds and the glass and the friction is pulling the glass out of your hands. It's very important to have the correct amount of water on the disks.
A good rule of thumb is to reduce the water flow on your disk until you start to see the white ground glass beginning to build up on the edge of your disk as you work. Then use a little bit more water to get that buildup off the disk as you work. That's generally the right amount of water for each grit. You'll eventually get the hang of which disk requires which amount of water.