my car idles perfectly fine but when my car is coming to a stop and my foot is on the clutch the rpms go up and down between 700-1300 but when my car is completely stopped it idles perfectly fine..just only when my car comes to a rolling stop and out of gear...what can this be? vacuum leak?....

my car currently has a dead altenator that i am replacing tomarrow if that has any effect on the rpms doing this

 

It could be electrical issues, IDK, replace the alternator and report back