DC spike via the barrel connector - poorly Uno!!

Hi. This is a shot in the dark!! :-)

Bought a 12VDC plug top power pack. Used it on my Uno-compatible board with no problems...after unplugging from wall and later plugging back in (with Uno connected already) the Uno LED's lit up momentarily then went out.

Investigation with a multimeter found that there's an unknown voltage spike from the PSU when you 1st plug in (Grrr! @£$&*£^%$&). So, when I 'try' to power the Uno now, either by DC plug or via USB I get upto 3-4s of life (even initial USB acknowledgement at PC) but then goes dead thereafter.

Is there any chance of bringing this back to life with a couple of component replacements? I guess I'm wondering if any experts would like to give their 2c as to best components to swap out...my initial thought was the U1 (NPC1117..) regulator but is this also in-circuit when powering from USB??

Thanks in advance.