Model C?? Or Model D??
![Confused :?](./images/smilies/icon_confused.gif)
The "C" has little grooves that the damper lever is supposed to lock into while you row. You must REALLY be rockin' and rollin' during your workouts for the lever to slip completely out of the groove on a Model C!
The Model D, on the other hand, was made without the indexed (so to speak) grooves. For all intents and purposes, you can move the lever on the "D" to any increment of any level. For example, mine is set to around 4.5 (best as I can tell). I can understand this slippage problem you're having on a Model D more than I can on a Model C.
In any case, I would take this up with ConceptII. Perhaps they can offer you a more graceful fix than duct tape.
Good luck.