I've seen this before; Are the lug nuts tapered or are the the shoulder type on the shouldered type? You can have a problem of the lugs bottoming on the stud...............the wheel is on there and the lugs tighten up to the proper torque BUT there is a couple of thou clearance...........the wheel feels tight but isnt. It only takes about 30 ft at speed for all the lugs to spin off. I caught this on my 1200 when using a differant set of rims, something just didn't seem right when I lowered the car off the jack........pulled out of the garage to put the car on the trailer , just happened to check the lugs....sure enough they were loose already.

Tom