There may be a timing problem in the Apple IIc motherboard.
Check location UC7 on the motherboard for a "black" (not silver) LS161 integrated circuit. If this component is black, then this board has a timing problem so the motherboard should be replaced. Refer to the service programs binder to see if the repair can be reimbursed under a special warranty program.
If the component is silver, then the board does not have the timing problem.
If the above cure doesn't fix the problem, search on GTS and the product's name to obtain a general troubleshooting procedure.