The disabling detailed stuff happens if your framerate drops below a certain threshold for too long.
When does the framerate drop and it loses the detailed settings? Is it when the game first loads, within say, 15-30 seconds of starting? If so, after you get the message and it disables them, can you reenable them and have it run fine?
If so, I had a similar symptom where as the game loaded my framerate would drop, disabling detailed water and terrain. Once the textures had fully loaded and the map had been updated, I could re-enable detailed water and terrain and run at 60fps. From what I could see it appeared to be an issue with loading everything on startup. Once stuff was loaded, it ran fine.
I believe it boiled down to my settings basically being right at the edge of what my system could handle. It was a couple years ago, but if I remember right, knocking shadow texture size down a notch or disabling the shadow smoothing checkbox made the problem go away.
One quick and easy thing- When's the last time you defragged the hard drive? That might help if it's as I described it above.
YMMV, good luck.
Wiley.