Saving latest (since sync) data when the power supply fails

Hi,

my iPhone4 has entered a failure mode since the batts went to zero mid call.

It's now in a restore loop - apple logo only then reboot cycle - (BUT only when powered by a pc usb). When powered by an Apple charger the normal boot up occures and completes with the iTunes restore sync picture. It will not turn on at all on batteries alone.

Apple and I have checked the battery, its fine and sitting charged at 3.9V. The additional charging on mains allows the iphone to draw 1A not the 500mA on usb - this makes a difference to the turn on behaviour as above i believe. I tried making special cable to transfer from mains charger to pc usb after the device was 'on' but this doesnt help. The cable works in forceing the data lines to 2.5V and 2V to indicate fast charge and forces the iTunes Restore mode but the pc usb does not reconise the device, sees a device but cant tell what it is - maybe another problem also or just the fact that the breadboard cable cant communicate at the speed required. Replacing the battery has no change.

I have tried iRecovery under various device status. I can communicate with the iphone and setenv, reboot etc. The standard setenv/save/fsboot solution to the restore loop doesnt work, nor does setenv/save/reboot. iRecovery reports 'gas gauge SWI low' problem.

The data will be on the logic board I guess in flash so my next option was to simply swap out the logic board into a new iphone and hope the device turn on problem resides in the power circuits away from the logic board.

Three questions please:

1) Has anyone seen this different behaviour based on charging option before and knows a fix ?

2) Anyone think of anything else to try before a logic board stripdown, maybe 'gas gauge' related ?

3) Is the logic board swap going to give me my data back and eliminate any faulty charging and power circuits ?