Need help, folks

I left my my nüvi 3790T unused for a couple of months, the battery almost zeroed.

I recharged it to 100%, all seemed fine and I started using it in may car normally.

However, when in normal operation, all of a sudden pops up the message EXTERNAL POWER LOST, TURNING OFF IN 15 SECONDS.

It turns itself OFF and, after a few seconds, it is back on again. I tried unplugging and plugging back on on the cigarette ligther (which I use for other devices, so it is OK).

The device was never dropped to the ground, was stored in a case. I'm planning a long trip and would like to have it in good working condition.

Does anyone has any idea on what can I do ?