I hope erik sir will spare me for my shortcut spellings.
the simple way to "be spared" is not to use them.

The keys in my setup are on a ebonite panel which is connected to the main board through a interface cable. the 8279 is used as keyboard interface
Now you mix in a 8279 and it might be helpful if you, since this thread is a mile long that you would pinpoint exactly how the uC "get hung". This is a tedious process because when it is "hung" it, most likely, is off somewhere that is not in the area where the problem started.

basically there are 2 possiblities
a) the ESD makes the uC go off to "somewhere unknown" non-related to your code
b) the ESD or something else makes the uC go off to "somewhere unknown" because your code can not handle something such as 2 interrupts close together.

if the cause is a) the nothing but ESD protection will help. If the cause is b) you may not even have ESD as the cause. I have seen "ESD problems" that were not. Many problems that are not ESD (forgetting atomicity as an example) will appear so random that they may falsely be considered caused by ESD. I do think that your problem is ESD, but maybe the ESD make the processor get hung because you do not handle a situation (such as an interrupt race) that coule be handled by "perfect" software.

So, you have two possiblities a) find out what actually makes tha processor hang when ESD comes around and fix that or b) get rid of all ESD. I recommend you do both.

Erik