Some 13-14 Focus ST models, both modified and stock, have experienced issues with rough idling, stalling, and overall bizarre engine behavior. This may or may not result in a diagnostic trouble code which makes these issues difficult to troubleshoot and diagnose.
The cause of these problems was determined to be a defect in the wiring splices used in the main wiring harness. As a result Ford first issued a TSB (13-09-07) but shortly after they issued a recall in an effort to fix those driveability problems (Safety Recall 14S17). If your vehicle was one of those impacted by this issue and was not yet repaired, take it to your local dealer to get the recall performed (don't forget to uninstall your Accessport first so they don't flash over it!)
See if your vehicle was affected by plugging your vin into the database.
Safecar.gov
TSB paperwork
Information related to the recall.