2000 Ford Windstar Question: my van started jerking this morning while driving down the road

i was driving , noticed my belt squeaking this morning when i started my van , then while i was driving around 60 mph it started jerking did this a few times back to back then i slowed down to about 45 mph and it did it again , when i sped up it did it a few more times then did not do it again all the way to work .... anyone else have this issue , or know what this is ??