Mark the spokes you have to adjust the most. If they turn out to be the same ones most of the time replace the nipples with brass ones. See if that works. I hhe had some old Alex Rims that I once had to replace all of the spokes on.

I have used Mavic and Dura Ace low spoke count wheels and normally they require more tension to start with than higher spoke count wheels. Stapfams advice about custom wheels has worked for me but I still have several sets of low spoke count wheels. On most factory wheels the nipples are aluminum and under tension it seems as if threads lose their ability to hold tight under tension. Whatever the reason replacing the nipples has worked in better rims for me. Just a suggestion.