The chain is skipping or running roughly.
If caused by stiff links in the chain you should clean and lubricate the chain. See Chain Lubrication. If the chain continues to skip, the machine should not be rowed and you should contact Concept2 immediately.
If caused by a worn out sprocket, you should replace the sprocket. See:
- Sprocket Replacement (RowErg, Model D and E Indoor Rowers)
- Sprocket Replacement (Model C Indoor Rower)
Note: When replacing a chain or a sprocket, Concept2 highly recommends replacing both the chain and the sprocket at the same time due to how these parts wear together over time.