Thanks to all who supplied possible solutions to my problem. Ultimately I used 4.7k pullups, changed the power supply to a highly regulated and filtered 5volts and used the suggestion to place capacitors across power to each chip and at the entry points of the supply to the board. I haven't had any reliability/stability problems since! Also, thanks for the web pages that were supplied, I've been looking for cheap sources for small all-in-one motor driver packages and will probably be using these in future designs.
Thanks again,
Andy