Chan HT

Have a set of EBC Greenstuff pads fitted to the front and after breaking in period,found that steering vibrate during heavy braking.A friend told me this could be caused by not using grooved discs as hot gases is generated between discs and brake pad surfaces.Never encounter this problem with original pads.
Fitted a set of grooved discs and after 500km,did some hard braking,vibration is still there.Fitted back the original pads,steering vib is gone.Anyone have the same problem or is this normal with the Greenstuff?
Thanks in advance.

Chan HT