Over Voltage? (Or Not)

Now that you have it working you can troubleshoot some more if you want.

First, leave the ground hooked to the battery & move the positive back inside the car where you had it hooked up earlier. Does the stutter come back?

Second, leave the positive hooked up and move the ground back inside the car where you had it hooked up earlier. Does the stutter come back?

This may tell you whether it is a power or a ground problem. You may need a long piece of wire.