98 Buick Regal

98 Buick Regal has intermittent fan blower motor failure; fan sometimes won't run for heat or AC, at other times the fan will work without any problems for days/weeks. When fan doesn't run the headlights may also flash on and off but not all the time, when this happens you can hear a relay clicking sound. The control unit with the 3 selector switches for fan, AC and temp. has been replaced along with the relay mounted under dash against the firewall. The car has the manual AC control system. Thanks for any suggestions you can offer.