I now think the problem was not dead spots, but moments of exessive current draw. Run time gets cut in half, Z-car has trouble giving power to it, but a thor clone sometimes gets it to run decently. This motor drains the battery faster in any car I put it in.
Run time with a Z car charged twice was 8 mins (stock 2.2 Z motor), and 4 to 5 mins with the 2.6 Bit motor.
Since I've only tried one motor, I'm not sure If all motors act like this when used in Z-cars, or I jus happened to get a dud
