Arduino Mega 16U2 will not self-install bootloader

[FishamanP]

I have a clone Mega 2560 equipped with a 16U2 that repeatedly loops the installer and seems to never successfully program.

• Installation sketch properly selected Mega 16U2 hex
• 16U2 does not have DFU-enabled bootloader, only USB-CDC
• Wiring has been verified from ICSP to ICSP header, ohming at <1ohm
• Same for 16U2 Reset to D10
• RESET-EN cut after Installer sketch programmed, in lieu of capacitor
• "L" LED blinks slowly while TX blinks at same rate (from countdown), then Arduino disconnects from PC for ~10 seconds. L begins blinking slowly once more while TX light blinks quickly, then board reappears
• If installation is monitored through USB CDC, the last line printed is "Attempting to enter programming mode ..." before the board disconnects
• Board VID/Revision don't change, even upon disconnection or rebooting Windows
• Shorting 16U2 Reset twice simply restarts Installer sketch, indicating HL2 was not installed (correct?)

I would appreciate any insight you can offer.