Temperature control

Cooked my first rack of spare ribs. I thought I had my temperature stabalized at 250 degrees. Temperature stayed at 250 for about 40 minutes. I placed two racks of spare ribs into my large green egg. After placing the ribs on the temperature dropped as expected. However, after an hour and a half the temperature climbed to only 210 degrees. I thought the temperature was supposed to stablize to 250 after about an hour. Am I doing something wrong? I ended up adjusting the bottom and top vents to get it to 250. I'm a newbie, just want to make sure i'm doing things correctly. Any suggestions please??? If I don't figure this temp thing out I will use my Auber temp control device on a permanent basis.