When I had this problem, it was the passenger seatbelt tension sensor. It's where the seatbelt mounts to the floor. You'll see the wires that run to it when you tip the seat ahead to get people/stuff in and out of the back seat. The wires almost always break inside the electrical insulation in such a way that it can't be repaired. I got pretty good at putting the car in neutral, wiggling the seatbelt, shutting the ignition off, opening and closing the door to reset the retained accessory power, starting the engine, and putting it back in gear without losing much speed... The fix is to replace the seatbelt assembly.
The second time I had this problem, some of the wires for the passenger seat sensor got caught on some sharp metal under the seat and sheared off. That wasn't an intermittent problem though. That was permanent, until I soldered them back together.