1997 Dodge Neon Idle troubles

I have a 1997 Dodge Neon that I just bought. The service engine light was on when I bought it, but ran good (O2 sensor I figured). I just replaced the alternator belt, front brakes, change the oil, filter and air filter. I disconnected the battery terminals and cleaned them. Now the normal idle about 1,000 will constantly race up to about 2,300 every 5 seconds and then come back down. It does it in both park and in drive. I didn’t disconnect anything else that I know of and don’t have any idea why all of a sudden it’s doing this. Thanks for any advice you give me.