extreme dropouts-one particular song?-solved-moved project files to other HD

If your hard drive performance is maxing out, drop-outs make sense.  I don't think SI or Session Drummer could cause such havoc, unless the samples are streaming from the same drive as your audio files.
 
Check for the SD samples under:
     C:\Program Files\Cakewalk\Vstplugins\Session Drummer 3\Contents
And SI Bass under:
     C:\Program Files\Cakewalk\Studio Instruments\SI-Bass Guitar\Multisamples
(Replace "Program Files" with "Program Files (x86)" if not found above)
 
If the samples do reside there (on C: drive), next question is: where are you Sonar audio files?  Do you have a separate drive?

Indeed, definitely keep your project files (if that's what you meant by song files) with your audio files.  Ideally, each Sonar project should look something like this:
 
D:\Cakewalk Projects\My Song
     ...This is where your project files (.cwp) reside
D:\Cakewalk Projects\My Song\Audio
     ...This is where all the Audio files reside for the project.
 
The "My Song" folder is completely portable and backup-able on its own, as it contains all critical Sonar files.  The main thing is that the D: drive (in this case) is not used for any function other than storing project files and streaming their audio.  You can store other static data on this drive, but nothing that needs to be accessed in realtime by ANY program.