Why does my check engine light started to turn off and on

There are many reasons that can cause the engine check light to illuminate and then go out. For example: an engine coolant thermostat that is sticking partially open, causing the engine not to reach operating temperature, may cause such a condition. Since the check engine light has come on, the EEC IV engine control module may have a code(s) in memory that can be read with a code reader or can be output to an analog voltmeter so an observer may count the needle sweeps to reference the stored code(s) (if any). The EEC IV (OBD I) "onboard diagnosis one" is fairly crude, so that conditions that can cause a check engine light illumination may not be stored if the condition corrects its self. The ten-pin (salt and pepper shakers) wiring harness connectors between the engine wiring harness and the body wiring harness could cause the check engine light to illuminate if making a poor connection. Ford had a recall for such a condition. Your best bet is to find an old Ford mechanic with EEC-IV experience since these older systems are not as helpful regarding diagnosis as the newer OBD II systems younger mechanics are more familiar with.