5m is much, much too long! If you have a look in application notes of 74HCMOS family, it's advised to keep copper traces shorter than about 20cm! So, without additional measures it's impossible to achieve reliable data transmission over 5m. Reason is very fast rise time of 74HCMOS output. This results in heavy crosscoupling between lines and in heavy ringing at the ends of cable.

Besides what Mahmood supposed, I would recommend to insert series terimination resistors of about 50Ohm at outputs of 74HCMOS. If it does not help, try to increase them a bit, may be up to 220Ohm, or so. But, anyway, drastically decrease cable length.

Kai