Garmin GPS III Plus initial startup screen just flashes on then off

Like so many others I am quite fond of my old GPS III Plus but now have a problem with it. It hadn't been used for a while but I decided to fire it up. Initially the screen was incomplete with missing columns of dots but they are all back now. Read on various sites about the internal battery so changed that out. New battery is putting out 2.9v now as opposed to 0.75v of the old one.
When I power up on batteries, car lead or using my mains adaptor the initial "splash" screen comes up with Garmin copyright 1998-2002 but then just goes off instantaneously. No combination of other buttons when powering up seems to overcome this.
However, if I press the Enter key when powering up I am taken to a test page as per this pic below. If I press the quit or page key I can scroll through 3 other pages that just seem to be screen test pages. I haven't seen any messages regarding ROM or RAM failure. The test page below suggests that everything is working ok I think...?
Anyone have ideas as to what the problem might be or how I overcome it? Would like to bring the unit back to life as it is fine for my needs.

 

Hi Nuvi Neubie, It behaved like this before battery replacement which was why i went down that route, full of hope.
I haven’t done any file transfers with it. Certainly not recently and can’t remember doing them when i got it back around 2002. Only have a serial cable connection for it but no pc with a serial port these days.

 

Hi Nuvi-Nebie
Thanks for the advice. I have ordered up a serial to USB converter cable so will see what I can do if anything.
cheers