well burning out is one reason,but also the motor movement was quite vibrating and uneven.iam not sure whether its bcz of the PIC or software.
and TIP122 is logic mosfet.can i connect it after bias resistor or do i need any transistor in between?
and what do u think of that CD4020 and CD4516 design.

and if possible pls share ur design.(circuit at least).later u can go for some detail.