1990 Nissan d21 pick up stalls while driving and slowing to a stop runs fine at idle?
Fuel pump is going bad or the fuel filter is plugged near solid.
Are you going up a hill? Did you take your foot off the gas and is the compression of the engine slowing you down? Are your brakes sticking? Is your car getting the correct amount of air to fuel ratio? there are lots of reasons why a car would slow down while you're driving it. start with the easiest.