I've lost track of how often I used to say defrag your HDD. For fewer drop outs and overall better Sonar performance there are a few things that should be done regularly.
1. Defrag the HDD.
2. Clear the Picture Cache at least monthly.
3. Use Per Project Audio Folders.
4. In large projects, minimize the number of audio clips by bouncing tracks to one clip for each track.
5. In large projects, finalize clip processes as soon as possible by Bouncing them.
Don't keep a lot of Take Lanes and FXs going at once.
6. In large projects, keep extensive clip editing to a minimum.
If you need to do a lot of heavy editing try moving the clip to it's own project.
Try not to load up your system by making it handle a large amount of extraneous unneeded stuff. Minimize the project as much as possible.