[joesam]

A series cart that shudders is almost always a power problem. Even though your charger seems to be working, something is starving the motor for power. Check all battery connections for clean and tight.

Then put a DVM on the entire battery pack and start to drive while watching the voltage. I'm betting you will see a large voltage drop when you push the GO pedal. It is not impossible that one of your new batteries is faulty.