car dies when coming to a stop

[7m90jt]

just got my 88 supra targa top back from the shop a week ago after having motor replaced, they told me the alternator was just about done for but i was still able to drive it for a day or so before it went for good. i replaced the battery and alternator yesterday but when ever i am driving and shift out of gear to neutral above 25mph the rpms drop below 500 and the car shuts off. it wasnt doing this before i changed the alternator and battery. i didnt see any senors unplugged or loose connections on the battery or alternator(i made the belt as tight as i could but its still is not as tight as it should be ,so could that be a problem?, not really sure what to check now so any and all input is great, thanks.

car is a 88 7mgte

 

[Nick M]

Why do you shift to neutral when going 25 mph?

 

[7m90jt]

Why do you shift to neutral when going 25 mph?

That's only when coming to a stop, if i shift out of gear while coming to a stop any faster than 25 the rpms drop to low and the car dies. Does that make more sense? I mean i can down shift until i am almost to a stop but normally once i see a stop light ahead i just put it in neutral and slow down with just the brakes