What analog pins are you using and does it still occur if you change them?
I use all the pins and still occurs if I change them. All the pins have the same behaviour.
try to not use an external power supply and see if that makes it work. I just got light sensor / temp sensor / ping and lcd working properly by not using an external power supply, but by using the arduino 5v. I was also previously getting 1023 (=5v) reading before I switched. do you have a photo of your setup?
I'm not using a external supply right now, I think should be a possibility. Iwill try to send a photo of the setup in the next days...