Two possible causes:
1. Power Supply RFI interfering with the disk drive's ability to read data from disks.
Cure: Check the Technical Procedures "Power Supply Retrofit" to determine if the unit has a Type 1 power supply. If it is Type 1, follow the procedure to install the shield P/N 612-5004 onto the power supply.
2. Color Monitor RFI interfering with disk drive's ability to read data from disks.
Check for head shield on internal disk drive. If missing, install per March 1986 Technical Procedures update. A May Service & Support Notice will explain how to obtain reimbursement for this procedure.
If this cure does not resolve the problem, search the TIL for Apple IIc and Troubleshooting.