You are going to find that newer microcontrollers that are designed to run faster and that run at lower voltages are going to be more and more sensitive to spikes and GND noise.

The solutions to noise spike problems are not going to come from entirely cook book type suggestions and guidelines. It takes work to use test equipment to find the problems and then make changes to eliminate those problems. Of course as always, previous experience with such problems does guide one how to design carefully on the next project to try and anticipate as many issues as possible and avoid the processor lockups in the first place.

I would also like to comment that if your current design with the 'F124 @ 3.3V is having problems "most of the time" with upset that it is likely that the earier version with the 'RD2 @ 5V was also susceptable to upset "some part of the time" and you just had not seen it yet.

Michael Karas