Bicycle Mechanics - Slightly loose handlebars - what to do?

Over time, my road handlebars rotate forward a bit (after 10-15 rides). I have to loosen the clamp bolts, rotate them up, and re-tighten the bolts. I tighten them pretty good.

This is an “older” ('97) threaded headset/stem/bar setup. Ultegra headset, aluminum stem, and probably an aluminum bar (?). The forks are carbon. I'm a little worried that there may be a crack in there (but it feels fine). Might just be a little stripped. I think there is only movement on big bumps. I can't move it by hand when it is in the stand.

Is this something I should worry about?
If so, what is the best fix (price conscious)? New bar and stem with a threaded to thread-less headset adapter?

Cheap fix: cut a piece from a soft drink/beer can and use it as shim; that may work unless bolt is stripped. Also could try using some Loctite.

Mark the position of the bolt. Check it after 10-ish rides. If it's not loosening, but the bar is rotating, you're just not tightening it enough (or you need zona's beer-can shim). If it is loosening, you might still not be tightening it enough, but that's when you know you need the loctite.

This happens to me (other way, though; weight on the brake hoods makes it rotate backward). I forced it once when I hadn't sufficiently loosened the bolt, and it stripped the bit of knurling on the OD of the bar (or was it the ID of the stem? I forget...). One day I'll get a new bar (stem?), or maybe I'll try the can-shim, but lately I've just been adjusting it every now and then.

You may have a mis-matched bar and stem. They come in various diameters depending on the country and manufacturer. If the stem is a couple of thousands oversize for the bar, you can tighten the pinch bolt all day long and still not get the clamp-up needed. An LBS could probably measure the stem id and bar od for you.

You could rough up the contact surfaces. I would always use Loctite.