Common problem with the engine in that car. There are a multitude of potential causes, so it really cannot be diagnosed over the 'net with the information you've provided. I suggest using the search function; there's plenty of accumulated reading on the topic. Start with a search on 'm104 idle surge' or something similar, then read through the resulting threads.

Because your car is a '96 it is OBDII compliant. Most of the cars discussed with this problem are older, not OBDII, and hence somewhat different. You wont be able to pull error codes out of the computer using the $5 Radio Shack homebrew reader. Rather, you'll need an OBDII reader. As an alternative, many auto parts stores will plug in and pull the codes for no charge.

- JimY