Y does my car shake when not acelerating in park.?

I have a1996 mustang gt 5 speed standard, when I have it in neutral and not pressing the accelerator it stays a bit less than 1000 rpm but then is goes lower and it acts like it wants to turn off but then it kicks back to 1000rpm, I'm thinking its the spark plugs but I want to make shuts before I to and buy anything.