Idles problem

When i first start my car it starts idleing up and down from 3000 to 2000rpms after about 5-10min of idleing it stops then goes down to 1500rpms. It seems to do this in warm and cold weather. Someone told me to buy a new o2 sensor, after i installed it it work for that day then started idleing wierd again. Anyone no whats wrong???

thx

Clean your throttle body or check you throttle position sensor.