Chris and Aye are correct of course but the FIRST thing to do is check the connectors for the front speed sensors because it is a two minute job. Follow the sensor wire from the front hub into the engine compartment and along to the front strut support housing. You will see the connector there. It must get oxidized enough to lower the voltage a bit; thereby giving a low signal when the system is calibrating itself when you first drive off after starting the car. A differential signal warns the ABS that all is not well and it disables itself.

I had the same intermittent problem ever since I got my 1990 two years ago. I cleaned the sensors, took off the front hubs to clean the sensor wheels. No improvement. I thought I had checked the connectors but I guess not well enough. The other day I checked again and lubricated the connectors with electrical contact grease. Voila! Problem solved. Rock solid ever since.

Bill