Stumbling?
 

My car has a problem that I cant diagnose. It started last weekend, Between 1800 and 2500 rpm or so the motor stumbles and the tach needle moves back and forth. If you give it more gas and go above 3k rpm it smooths out and runs fine. It also runs fine at the typical cruising rpm of 1300 or so. It only does it sometimes maybe once a day or so. Everytime I try to show my dad the problem it always runs fine. I dont know what would be causing it, i'm assuming something in the ignition, what would you reccomend looking at first to solve this problem. Thanks.