Try to imagine such a two layer board with two DIL packages at a distance of about 10cm. Imagine the one sends a signal to the other, without the signal trace running over a solid ground plane. Then a current loop of an area of let's say 10cm x 5cm =50cm^2 is formed. This loop not only emits a magnetic field, but also recieves magnetic fields from external EMI sources, while the induced voltage is proportional to the loop area.

Now, the same situation, but with SMD packages and a solid ground plane for the ground return current. The area formed by the loop current is now about 10cm x 500µm = 0.5cm^2, assuming a four layer board.

So, in this example the proper four layer board design is about 100 times less sensitive to external magnetic fields, then your two layer "solution".

Do you still think your problems are caused by the microcontroller?? The microcontroller is only the victim of your improper design, not the cause.

Kai