When I got extra grip screws, along with a few other spares (recoil springs, particularly), Maria suggested using some blue loctite on the screws if I did take them out. That didn't come as a surprise to me, since I've used blue loctite on several grip screws for other pistols. This may well be what you need.
A few pointers, if you use it.
1. MAKE SURE THAT IT IS BLUE LOCTITE. Blue is the low strength version that allows fasteners to be unscrewed readily. Any others will be much too strong and you may never get the grips off again in one piece.
2. Use it sparingly. Just a very small drop is more than enough. I know some that put a drop on a small screw like these and then shake the screw to get all but the last bit off.
3. Clean both screw and frame hole first. Isopropyl alcohol on a cotton swab works pretty well the first time. Later you'll have to clean the old stuff off with a toothpick, or similar, before reapplication.
A second option is a similar small dab of clear fingernail polish on the screw. The steps above apply equally to that, except you can clean it up for reapplication with fingernail polish remover.
