Rear wheel/axle slipping

I'm having no luck finding a solution to the rear axle problem on my second SS. I've got a lovely Raleigh Pursuit frame with horizontal dropouts and some very lovely Fulcrum Racing 7 wheels with a freewheel rear hub. Had no problems with the chain line or tension - but the first time I stepped on the pedals, the axle slipped and pulled the tyre across to rub the chainstay. I assumed I hadn't tightened the allen-QR skewers enough (new, "aircraft grade alloy", apparently), so I realigned the wheel and tightened them further - but found that I could easily pull the wheel out of place by hand. I checked Sheldon, checked LFGSS, decided I was being a wuss, REALLY tightened the skewer, and promptly snapped it, sending the acorn nut + snapped portion flying across the room, where it made a small dent in my wall. I went back to an old cam-based QR skewer, and tightened it to the point that I couldn't give it any more (bearing in mind I rock climb to 7a+) but still, the wheel can easily be shifted by hand. This QR system is the same I used on my first SS bike (also with horizontal drops), and I cycled that for a year without any problems at all.

Any help, please? It's a bastard to get the rear wheel in and out of the dropouts, but no worse than any other bike I've worked on - I can't work out where I'm going wrong...

tighten it more or switch to a solid axle.

I had similar issue, the axle was slightly too long (1 or 2 mm). I spaced it out a little and the QR holds well under tension.

axle is too long - take a few mm off each end (carefully) with a file.

Cheers Will78 and Cranks Joe - it's about half a millimetre too long. Problem solved!

or get an axle tug thing