Not a lot of details in the above, so here's a few generalizations.
Power surges and switching hash caused by the motors gets into the microprocessor from trying to
use a common power source. Power the motors off a separate power supply that is able to provide
the current necessary to run all motors at full power.
Not having a common ground between the Micro-controller and the motor controller board.
Not using hash bypass filters on brushed motors. The usual is .01 uf capacitors one each from the
motor terminals to the metal motor housing and selected for a working voltage at least twice the
source voltage supplied to the motor
Grumpy Mike has a few good pointers on dealing with power problems:http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html