I agree with the others that you should look down the neck of the bottle for the physical inspection. Then use a bottle brush and cleanser/soap to clean the inside followed by rinse and sterilization. You have to clean and sterilize. Using sterilization without cleaning doesn't work because bacteria will cling to the sides. In addition, soap sterilizes as well.
If the infections persists, look into the other points already made about beer lines etc. If you are brewing outdoors or in the garage, it might be possible that you are picking up wild yeast.
The way to fix every issue is to pasteurize the beer after it is bottled as follows:
1. Place your bottles in your boiler kettle and fill with water about 3/4 way up the bottles side.
2. Open one of the bottles and place a thermometer in the bottle to track the temperature.
3. Start the fire on high and track the temperature. Your goal is to reach 165°F.
4. Turn off the flame at about 163°F and allow the temp to rise to 165° F.
5. Remove the bottles from the boiling kettle and allow to cool.
6. Recap the open bottle. BTW: It will foam during the heating, but don't worry it will be fine.
Cheers