Idling Problem

I just changed the air filter on my 91 Plymouth Laser 1.8. I also sprayed some throttle body cleaner in the throttle body while I had the air intake hose on. I put the new air filter in and reattached the air intake hose. Now, the car idles around 1400 RPM's and then it will drop down to a 1000 RPM's and continues doing this up and down idling every couple of seconds. Other than that, the car runs OK. The air filter was extremely dirty that I took out. A neighbor of mine said that since it was so dirty, the computer has to find the correct air/fuel ratio since now it has a new air filter in it. Is this true? If not, what is causing this up and down idling every couple of seconds. Thankx. Gary