Thanks both of you for your help. I upgraded to 1.3.15 and it appears to have solved the problem. Appreciated the advice.

Recently switched to Deluge (on Mint 17.3), and am extremely pleased with its performance. However, some multi-part torrents are failing when I attempt to extract them, all with checksum errors in one or more part. A forced re-check nearly always finds no errors, and seeding resumes. But the checksu...