Lpc2000

Thanks to everybody who replied, we'll check if the diode schematic 
will improve situation (never liked to make voltage drop with diodes 
as it vary with current consumption)

>You talked about the 3.3V supply and that things worked at 2.5V,
>however, you did not mention what was happening with the 1.8V supply.

1.8V is always solid 1.8V in both cases (code run or code not run), 
the observation that code always run with 2.5V power supply was side 
effect we found during our digging for the reason for the problem.
LM1117 which make +1.8V (core supply) have always sufficient voltage 
drop to ensure proper operation, just the second LM1117 which is used 
for IO +3.3V power supply voltage vary when we include Zener to the 
power supply (i.e. decrease Vin).

Tsvetan