I went with an off the shelf Dell with an I7 and 16gb RAM, but made sure to get a version geared for business without the bloatware.  It came with Windows 7 Pro and a disk for Window 8 Pro (now upgraded to Windows 10 Pro).  All I had to do was disable the integrated audio and install a video card.  It works really well with my 18i20 and it was a reasonable price.

townstra
I went with an off the shelf Dell with an I7 and 16gb RAM, but made sure to get a version geared for business without the bloatware.  It came with Windows 7 Pro and a disk for Window 8 Pro (now upgraded to Windows 10 Pro).  All I had to do was disable the integrated audio and install a video card.  It works really well with my 18i20 and it was a reasonable price.

Pretty much what I did. You can uninstall the unnecesssary crap like Intel management software in add/remove programs, or what I prefer to do, wipe and install windows from scratch. My Dell works very well indeed, and is also optimized for porno...

https://youtu.be/8yVM8KljypI

To OP try this...
 
Try deleting AUD.INI first (google for details).
After that,go to Sonar preferences and disable - allow arm changes during playback.
 
Ta.