Buck,

You might be fooled into thinking the processor might not be working, as it should when the problem could be a dirty incoming signal. Have you tried to clean the signal with a 74LS121 single shot? I spent many a night trying to figure out why things were not working as they should until an old timer took it upon him self to feed the troublesome signal into a single shot and demonstrated the results to us by no more failures. It made believers out of a lot of engineers.

Regards,

Charles Bannister