This has been going on for a year or so. I finally decided to mention it here. When I copy & paste an audio file (e.g. WAV) into Sonar (or import it, doesn't matter how I do it), the clip goes in at measure 4000 or so. I have to scroll to the right all the way, cut the clip, and paste it into the beginning of the track.

Hi. Have you tried to uncheck import broadcast audio at their timestamp, in Preferences?
All the best.

the default was changed in platinum, thus not doing this by default.
possibly you may have checked the setting by mistake, then again I have no idea what version you are using.
(nothing in your specs / or mentioned)
As ken said uncheck this option.
 
Always Import Broadcast Waves at Their Timestamp. Forces SONAR to ignore the Now Time when importing Broadcast Wave files. When this option is checked, Broadcast Waves begin at their SMPTE timestamp in your project
 
So you want to unheck this option in preferences
See more here:
https://www.cakewalk.com/Documentation?product=SONAR%20X2&language=3&help=Dialogs2.063.html

 

Also when exporting a file don't use Broadcast wav files unless you have a good reason to do so.