Tech came out to replace thermostat with customer supplied thermostat. Open arrival tech tested operation of system at thermostat, system came on briefly and thermostat rebooted. Customer stated that it had done that for awhile. Tech verified the compressor contactor was not shorted and replaced the thermostat. Tested operation and system cycled on correctly, however it is low on refrigerant. System is 28 yrs. old tech did see an oily residue on the suction line at the air handler indicating a refrigerant leak at the evaporator coil. Replacement of system is highly recommended. Tech quoted customer for 3 lbs of R-22, customer approved but tech only added 1 lb of R-22 at this time. Total charge is about 5 lbs.