I dont think it would be the head race bearings as I`v had that. You just get 1 knock as the weights shifts forward and thats it. No shaking.
As been said I would probably go with...
Caliper overhaul, pump out and clean up and grease (red rubber grease if poss) pistons so they can all be pushed back flush easily with your thumb.
Scuff pads with a fine file or paper, clean discs, gently re-bed pads.
Check you wheel bearings and that the spacers are in correctly.
Good luck!