I am working on making a tiny obstacles avoiding robot (with the picaxe 08m). The robot use small dc motors to move and a MaxSonar sensor to detects objects.
The problem is that when I set all pins on the picaxe chip to low, the motors movie in pulses, as if its somehow picking up the pulses from the sensor.
Are you sure that you are pulling the outputs low. Could it be that they a set in tri-state mode (floating).
I would put a resistor from the point between the existing resistor and the transistor to 0V. The size depends on the size of you existing resistor, but try with at value between 2k - 5k.