03 Dodge Caravan shudders

My 2003 Dodge Caravan shudders at high speeds ( 90 -100kms/hr). If I pull over, come to a complete stop and return back on the highway the shuddering stops. The last time - an hour and a half trip - I had to pull over at least four times in the first 40 minutes. After that it didn't happen again. I've recently had new tires put on and have completely replaced the whole braking system.