Hi,

Interesting problem!!

I would suspect a voltage surge on the 5v rail when powering-up. Have you tried increasing the capacitance on the 12v rail to deaden surges hitting the 5v regulator. I have seen some 7805 (not 78L05) regulators 'whistle' on their output giving peaks of 8 volts if the capacitance is too small.

Is there any big inductive loads powered from the supply?

The unused inputs can be tied to ground or 5v with resistors (1k).

Last week 4 PICs went south after passing a functionality test on the bench
Was this a test using the PCB but with nothing connected? Do the PIC's die with just power connected or on load?

If every PIC dies on the bench in different (but identical) PCB's then I would look at the board layout carefully. If you can post a picture of track and component side I will look at it.