Cannot install or update apps on LG G2x
I had the same problem with my wife's phone (LG G2x P999). Unfortunately you're going to have to master reset....

If it doesn't work from the settings, then reset manually (hold power button and volume down till you see the Android guy then let go) after that your phone will be reset and the downloading issue will stop. Read through these steps prior to resetting to ensure you don't lose anything you don't wanna....hope this helped you

http://support.t-mobile.com/docs/DOC-2521
should be the 5th link down where it says preparing for master reset