I have the same problem, download ends, installation process begins and when it is supposed to end ... after several minutes indicating that it is installing it automatically cancel the installation and starts the download again without human intervention.
I have this messages in the console:
27/09/12 11:43:10.464 installd: PackageKit: Code signature on /private/var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/Cleanup At Startup/PKInstallSandbox-tmp/Root/Applications/Xcode.app/Contents/Applications/ Application Loader.app/Contents/Library/Spotlight/MZSpotlight.mdimporter/Contents/MacOS/MZS potlight did not verify (invalid signature (code or signature have been modified))
27/09/12 11:45:03.580 installd: PackageKit: Install Failed: Error Domain=PKInstallErrorDomain Code=123 "The update was cancelled because the application integrity could not be verified." UserInfo=0x7fac7bc98080 {NSURL=file://localhost/var/folders/tf/t4c3dqzj30ngb702fq23n70r0000gn/C/com.app le.appstore/497799835/mzm.ftitpajc.pkg#XcodeMAS_iOSSDK_6_0.pkg, PKInstallPackageIdentifier=com.apple.pkg.XcodeMAS_iOSSDK_6_0, NSLocalizedDescription=The update was cancelled because the application integrity could not be verified.} {
NSLocalizedDescription = "The update was cancelled because the application integrity could not be verified.";
NSURL = "file://localhost/var/folders/tf/t4c3dqzj30ngb702fq23n70r0000gn/C/com.apple.app store/497799835/mzm.ftitpajc.pkg#XcodeMAS_iOSSDK_6_0.pkg";
PKInstallPackageIdentifier = "com.apple.pkg.XcodeMAS_iOSSDK_6_0";
}
Any useful information will be greatly appreciated 🙂