So basically I have the same problem . . Well almost, so basically I had been messing with my build.prop and everything was going just fine the whole time through out. So after I was done modding pretty much, well I was testing out different settings I can input to change my Spring LS990 to work with the T-mobile carrier, including having to enter into a "secret menu" through the phone dial, but ended up bricking it. So after I was done therefore, I went and copy-paste the backup I made of the build.prop, straight into /system folder. After applying confirmations and letting it do it's job, I gave it a reboot for the copy to take effect and that's where I ran into the problem of having a boot loop on the LG logo screen and being able only to turn it off by removing the battery. Also I'm able to enter recovery, use ADB (kinda), but keep getting denied access/permission/privileges to use other functions in ADB exept the "adb sideload" command, but even that results in mounting failure and signature+foot errors. Yes, I've been to numerous posts and forums, searching everywhere for info on my own repair and ended up here post this. Really appreciate your help, am willing to pay considerable amount for repair if extremely necessary and would also like to end up getting the phone to work with T-mobile if possible.