Truly, the dynamics that happen with your weight, climate, and many other factors effect the front brake. It really should be a matter of elimination. Forks loose? Brake bolts loose, pads dirty, not likely as usually just a squeal, not shuddering. Really either a loose headset, or loose brake fasteners. Over tighten one gently try it, if no fix do the same with the rest until the problem is gone. There can't be many factors, fork, calipers, or maybe the pads lined up really off center, but if a shop looked at it, not likely. good Luck, be safe