We have had a similar issue, although only on 1590s, not on 1550s.
In our case, the gateways have a connection to the customers internal WAN and the internet. Following the topology that we had in place when we were using 1400 series gateways, we used the connected the Internet to the gateway WAN port, and the customer WAN to the DMZ interface, with the site internal network connected to vlan interfaces on the LAN1 port. We disable the internal switch on these devices.
On 1590s with such a configuration, attempts to upgrade the gateway would do exactly what you described. The firmware would appear to upload correctly but after some time, the gateway would finally come back to the original version. We had a connection to the devices console port and we could see the boot messages. Effectively what was happening was that the gateway would go into maintenance mode and reboot five times to boot of the new firmware. After five attempts, the device would revert back to the original release.
The fix was to use another interface for the customers wan connection, in this case we used LAN8. Having done this we have not experienced the issue since. Why we never struck the issue on 1550s is because these devices do not have a DMZ interface hence we always use LAN5 for the customers WAN connection.
Hope this helps.