Bicycle Mechanics - tube slipping - how to stop?

Hi everyone
I'm having problems with flats with cut stem bases. I do my best to keep them straight, keep my tires aired up properly. If I notice the stem going angled, I stop to deflate, straighten, and inflate. It is happening on nearly every trip. (It is only happening on the front tire.)
Should I drill the hole bigger, or is there another root cause that is making the tube rotate in the rim? It's driving me NUTS.

I wrap some cloth sticky tape around the base of the valve stem which acts as a stopper, preventing the valve moving around, and also stops it pulling through the hole and tearing.

Maybe the tube is for a narrower tire, and is thus not filling the cavity as well as it should? Just a thought. Maybe the tube's for a smaller diameter tire (26" tube in a 700 rim/tire, e.g.)?

Assuming that you are maintaining the correct tire pressure (and it's a schrader valve), you could consider using presta valved tubes, which have a thread all the way own the valve stem, with a ring that you can thread down.

Continental makes schrader tubes that have a lock nut just like a presta tube. If nothing else that valve will NOT move.

Thanks everyone for the great help.
I found out the problem-my tire was worn out. Evidently, it had stretched and it was also turning on the rim. I figured it out when I realized that the tire was coming off the rim way too easily when I changed the flat for the third time this week.
I bought a new tire and that sucker is staying as straight as can be.
Again, thanks for your help.

Are you using decent rim tape. Velox cloth tape is cheap, easy to fit, lasts for years and prevents a whole range of inner-tube problems.
Also check if the the valve hole of the rim has a sharp edge. Emery cloth can take the edge off.