Neilanrx7

So I have an 86 n/a. The other day I was driving around and my car died when I had the Clutch in (car was in nuetral) and when the rpms were dropping to idle it dropped to 100 rpms and died. Now whenever the car is idle it drops to 100-ish rpms and then jumps back up to 800(idle) then back down to 100. It does this a couple times until it finally dies. It still drives fine the car just dies whenever in nuetral unless I keep revving it up. Pls help me out I have no idea what's wrong.