This has happened previously, was solved, but I have forgotten how. I can't address the matter on the Sonar forum because my account is inoperable, and after a month or so, Cakewalk has declined to provide information or assistance.
GPO & Sonar 4, 4 section string plus solo flute and bassoon. Max cpu is about 55 %. But at any load, all the strings stop playing, can't remember bassoons. Solo flute continues. In about 2 seconds, playback returns to normal. No error or warning. Not always same place. No differnce between VSt and DXi.