Daniel,

It is difficult to suggest a solution since the information you provided is not enough.

However, if you have done as Mehmood suggested, and still facing problem then it seems a software problem.

May be your software doesn't include every possibilty for inputs (from optocouplers)

Definitely when your power supply is disturbed, the o/p of optocouplers gets changed.

Re-consider the possibilty of any bug in the software or a short circuit from optocoupler's output to any input of either serial to parallel converter or the latch.

Good Luck

IA