weird, I just posted the exact same problem, this is happening on our 1993 Del Sol Si.

I recently did the brakes, I didn't bleed them, I would think if it was just air in the line the brakes would be spongy, but this is a complete loss of pressure. Only happens at stop lights, and not all the time. If you figure it out let me know my g/f is scared to drive it.

I have a Haynes manual and see if this is covered in the brakes section and post back.