One thing i forgot to tell you is that both serial to paralled converter and latch ics are not in the same board containing controller.I'm using shielded cable to provice connections between them and one end of shield is connected to ground.
How long is your cabel?
Once I had this problem doing eprom programmer from parallel port of a pc to the board, I made the cabel as short as possible and connected 10nF caps to the board side for each pin of the connector.
Your cable acts as antena and might be the main source of noise.
Regards
Mahmood