Mercedes Vario 814 braking problems?

My 1999 Vario 814 had new discs and pads all round, one new front caliper, plus new rear hubs, fitted by a main dealer about 18 months ago. On a recent run back from Spain I had a problem with the front brakes overheating on long steep descents accompanied by very severe front end vibration that felt like one front brake grabbing and releasing. The brakes were fine in normal use.
I went been back to the main dealer who said that they could find no fault except that the new pads were badly burnt and that I must have been riding the brakes. I wasn't, and had done the same trip, before all the work to the brakes, with no overheating or vibration problems at all!
New pads were fitted again but I suspect an underlying cause. Suggestions on what to look for appreciated... I don't want to have brake problems next time I come down a mountain road!