P1421 - Secondary Air Injector Valve Circuit Short to Ground
Seems likely you have either a bid solenoid for the SAP vacuum circuit or possibly a wiring fault? If intermittent, might be a solenoid, but in any event, I would not expect this code to be related to any rough running, unless somehow your solenoid is stuck open? But that would be pretty easy to check by pulling the large hose of the SAP check valve on the passenger front corner of the engine.
Suggest you really look over the beginning of this thread carefully, as the most common problems are outlined there.