The area around the valve hole AND check your rimstrip. This sounds like a bigger issue than tubes.

Also, what pressure are you running? Too ow can be a problem.

Do you use that little nut that comes with some tubes? It can help stop the tube from creeping around inside the tire and cutting the valve stem at the hole in the rim (CAUTION: if you tighten it too tight, it can also pull the valve stem too much and speed up the process. Just snug it gently and it may help you problem).