DP83848C: DP83848 stops working after few month on field
The DP83848 absolute maximum for input voltage is 4.2 + 0.5V = 4.7 V.

The LPC1768 inputs are 5V-tolerant, and can withstand up to 5.5 V.

I think your problem is located in the power supply design. You use two pi-filters, and this type of filters can be extremely dangerous on specific power level rise times, with the circuit oscillating up to 2x supplied amplitude, thus killing your electronics.

Why 2-3 months? I would say coincidence, but looks like something in the reset pin input circuit being constantly worn out (e.g. protection diode), and finally failing with short circuit. Check all other pins to see if there're any other with the same symptoms.

Perform a simple test: connect scope to the 3.3 V power rail and see how power voltage behaves when you turn the system on, and turn it off - using your lab power supply and the one(s) used in the field. It would be great if you will see clear exponential increase up to 3.3V, but it may also happen that power fluctuates between 2 and 5 volts for several milliseconds.