> I have four 16C74's that have been programmed MANY times.  I have not used
> them for a while, but just recently thought I'd give them a try.  I loaded
> them with some code and they seem to work..... well sort of.  They would
> run for a while and then lock up for no apparent reason (they are driving
> an LED display) sometimes they wouldn't even start at all.  I have tried
> all four PICS with the same code, and they all do similar things, but I
> can't get them to fail consistently.  The only thing that seems to make a
> difference is how much I load the ports.  If I only fire a few of the LED's
> at once they seem to lock up a lot less frequently (they still lock up
> sometimes).  I have all of the LED's run through buffers, so I know I am
> not loading the port TOO much.  Is this a sign that I should buy some new
> PICS or am I missing something.

[1] Do you have all VDD and VSS pins tied appropriately?  If you remove the
    PIC, can you measure 5 volts on all pairings of VDD and VSS pins [e.g.
    5 volts from VDD1 to VSS1, VDD1 to VSS2, VDD2 to VSS1, etc.]?

[2] Do you have a suitable-sized bypass cap?

[3] What is your total current sourced and sinked?