inductance - Adding a ferrite bead to source of mosfet (smps)

We are designing a SMPS (ultra low noise) based on TI schematics http://www.ti.com/lit/df/tidrgc2/tidrgc2.pdf (5V, 15W quasi resonant)

In order to reduce EMI (we have some ringing across Mosfet when its turned off) we added some ferrite beads before the R4 and R5. (330 mA, 600R/100MHz, 480mOhm DCR)

Of course we modified the R4 and R5 (we subtracted the R of the ferrite bead). We also added a FB on gate of mosfet (same specs).

However while ringing decreased substantially , every time we push to 15W (load) our unit goes bad. It works fine at 5 W.

I read that inductance on the mosfet source is a bad thinng in SMPS. Can it be the root cause (inductance of the FB)? Am I missing something?

update:
We removed the ferrite beads as instructed. Will try a different approach.