Use Odin3 v 1.83 or similar. Download a .tar file version of a kernel that is compatible with what firmware u r currently running (example: use KK4 Corn Kernel v7.0A if ur firmware is gb i897uckk4). Open Odin, select the PDA tab, navigate on ur pc to the .tar kernel, select it, connect ur phone to the pc in download mode and select Start. Done. Reboot into cwm recovery.
Or since u have Root...copy the .tar kernel to ur phone's internal sd and use SGS Kernel flasher from the Play Store. Done...again.
Fyi, Don't use Rom Manager---it doesn't play well with the Captivate.
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