I had the same thing happen to mine once, turned out to be the cylinder head temp sensor on the front by the distributor. Nissan doesn't sell the correct one anymore, but we did a bunch of cross referencing at an old school parts house, and wound up finding one for a toyota pickup that fit and worked perfectly. Basically when that thing conks out it makes the fuel injection dump heavily so you get black smoke on acceleration, and finally clog up your plugs, and possibly washing the cyliders in the process. There's a procedure how to check the sensor in one of my old chilton manuals, which is in storage at the moment, but see if you can find it. Also another real quick easy check is to swap FI computers with someone real quick, altho in my experience, the car quits completely when those fail.