[Jon Stratis]

It's hard to tell exactly what the problem is, can you describe it?

Check how everything is wired. Can you guarantee that the wires from each motor go I to a separate speed controller, and nothing is accidentally crosswired?

Take a look at the LEDs on your speed controllers. You should see two light up for each side (note red/green for each!), Then running all of them forward you should see them all light up the same way they did when you ran each side individually. Do this without the motors hooked up to avoid having the breaker trip. That would rule out a code issue.

Are you sure your motors are going in the same direction inside each gearbox? If the polarity on a motor is reversed, you may be sending them in opposite directions, making them fight against each other.

Do you have a clamp meter you can use to measure each motors current while running? You may find that one or two motors are running "hot", helping you to narrow down the location of the problem.

Can you try running it without any belts? Misaligned belts or significant friction could cause armour issues.