Ok, I think I'm getting closer to an answer here. What is the "L" on the Leonardo mean? I ask because I have noticed the following:
Two 9V batteries were used. 1 was plugged into the ext_pwr slot on the motor shield (jumper on), and 1 was plugged into the Leonardo board (using a barrel plug).
At first, nothing happened, and it was stuttering, as usual. But then I noticed the "L" was blinking on the Leonardo. As soon it stopped blinking, the wheels began to move as programmed. And it did so for about 95% of the time fluidly, until the "L" started blinking again, and everything basically stopped. Then, the "L" went off, and all functioning returned.
I've tried this with various options, using the two 9V batteries, or just a 7.5V (5 AA) plugged into ext_pwr jack (but results aren't as good, as 1 tire will sometimes spin wildly fast).
When I only use the USB, the "L" blinks at the beginning, and then never does again, as the program runs fluidly.
So it seems that something is happening making the "L" light up at the beginning using the 9V batteries, and then again after maybe 2 minutes in, which stops the wheels from rotating. Then, after the "L" disappears, all is functioning again.
Any insight into what may be going on?