Why would a 91 Nissan Sentra 4-cylinder STD stall occasionally when coming to a stop?

Answer Its your EGR valve.

Had the same thing constantly happen on my '93. Its your EGR valve that's sticking. Nissan uses a plunger type valve that's when sticks stalls the car. Find the valve (looks kind of like a spinning top) and while your car is running reach under and using your fingers move the plunger. This will stall your car. Replace the valve and drive happily ever after. This fix is definitely a DIY as it only takes a wrench and or ratchet.