I bought a 1988 Bianchi Super Leggera bike with Campagnolo Chorus hubs and quick-release skewers last year. When I first rode the bike, I found that the rear wheel would gradually slip in the horizontal rear dropouts, particularly with hard acceleration or hill climbing. Even when I really tightened the quick release skewers as much as possible at the beginning of a ride, the rear wheel would end up tilted enough to nearly be touching the non-drive chain stay by the end of ten or twenty miles of riding. My local bike shop also tried tightening the skewer, and then simply shrugged and said that this kind of slippage in horizontal dropouts was an inherent problem with older bikes and heavier riders (I'm about 175 pounds). However, when I took off the rear wheel later for other adjustments, I found a simple explanation and fix for the problem. The Campagnolo locknuts that are used to hold the position of the cones on the hollow axle have two very different sides: a smooth end with flat faces that face inward towards the cones or spacers, and a serrated end that is meant to bite into the inner surface of the rear dropout (item 15 in attached diagram). The previous owner had installed the Campy locknut in reversed orientation on the drive side, which put the serrated face pointing in towards the cone and spacer, and the smooth/wrench flat outwards towards the dropout. That does make it easier to get at the drive side wrench surfaces as they poke up through the freewheel. However, it puts a completely smooth, rather than a serrated face, against the dropout. I switched it back around when I was servicing the hub. Voila, slippage problem completely cured, just from having the rougher edge against the dropout.