94 s10 blazer RPM fluctuates at idle?

In park the truck feels like it's going to die out and when I bring it to a complete stop at drive it feels like it wants to take out by itself and jerks. I just drove it around and it feels fine, just whenever it's stopped or at idle it the rpm flucuates. What could be wrong with it? I just replaced the fuel pump earlier today and it just started doing this. It's a 4.3, W code.