windshield wipers

[drew9000cse]

so i purchased new windshield wipers, the best ones i found Rain X at 8 dollars a piece, and i also put Rain X on my windshield. but now that i have these new wipers, and after they've been going for a good minute or two, they start shaking on their way down, they start bouncing. i notice that happens when a windshield is very dry, or at least it CAN happen. because when i have the back wiper on, i sometimes forget about it, and after the rain clears, it starts moving down the window very very slow(which is fine). but i just wanna know why it bounces after a while.