If Android starts failing to boot and reflashing is also failing there are probably defects in the emmc storage, but if you're able to boot a 3.14 image from USB the factory u-boot is still intact. Before attempting anything else I would use the USB booting OS to make a backup of the first 8MB of the emmc so that you have a copy of that u-boot, because although it's fairly simple to build bootable images for GXM devices, if the box has extra-cheap emmc chips (not unheard of) the fip sources might have been tweaked with custom timings (to deal with out-of-spec chips) and then it's unlikely you'll be able to find or recreate a bootable image. If the emmc deteriorates further it's then possible to erase emmc (short pins to boot from an SD card with factory u-boot installed). Worst case you might end up in a scenario where you need to boot from u-boot on a SD card but run an OS on a USB stick.
NB: Once you start fiddling with boot firmware and u-boot it becomes important to have a USB > UART cable to see the boot/console output. If we can see that output we can literally see where things break and what happens when we experiment. If we can't see that output, we're reduced to guessing where the issue lies.