To Mike, yes we finally got to what was going on. Persistence and pictures to prove what was what eliminated arguing.
Tumbel, good on you for making a makeshift rolling road, EXCELLENT! Never embarassing to show ingenuity or resourcefulness!
So, it seems that you are narrowing it down to one motor block.
It really sounds like mechanical (excessive friction somewhere) or the motor is bad. Motors can fail in interesting ways. It could be something as simple as brushes so worn that they act erratically, or shorted windings in the motor, which can act up when they get hot. (the insulation on the motor windings (wire) is very thin, so you can have damaged insulation that show up as a partial short when the windings heat, they are copper, they expand and can touch each other through damaged insulation.
The insulation on motor windings is a very thin coat of enamel paint, effectively. The motor current really seems to point to damaged motors, but it could be drivetrain binding, although binding usually is erratic, and comes and goes with each revolution.
Keep at it, you are progressing!
Greg