Alright so whenever I start my car up, it seems to idle a little under 1000rpm's and is kind of a rough idle. The lights and stuff are dim, but when I give it gas they are fine. I had just had the shifter cable replaced and before that, it ran just fine and it had not done this. It was sitting for about a week without being ran while waiting for the shifter cable to be replaced. What could this be?

*Note: it ran perfectly fine before it sit for a week when replacing the shifter cable. No signs of this problem at all until after that was changed. It's a 97 Monte with approximately 115k miles.