Hello All - did a few searches and see some suggestions for battery wiring and connectivity and maybe ECM connectivity. Car is 2007 XC70 with 250,000miles

Story - Driving at 40mph in extreme cold, 0F degree with -20F wind chill. Car suddenly bucks hard once, dash goes completely black for 1 second then comes back but clock is now zero’d. Continue driving while trying to assess what the heck just happened when within a minute get another hard buck. This time dash goes dark and stays dark, car stalls, and I have to wrestle it to the roadside and bring it to a stop with no power. I tried to engage the hazards to alert drivers behind me but no power. Tried to roll down the window to wave them around but no power. Dash is completely dark and car is not running but key is still in the run position as it had been while driving. Shifted to Park. I turned ignition off, removed key, reinserted and attempted to start car. Not even a click. Tried thrice more and still nothing. Removed key again and went to trunk to check on battery. Connectors firmly in place with no play. Jiggled main cables a bit to see if there was a Hot grounding somewhere and sparking but nothing. Got back in car and attempted to start and it fired right up with all dash and power accessories functional.was able to drive 15 miles home without further incident.

Backstory - had just installed brand new battery 2 days prior. Was having intermittent hard start issues where battery would spin the motor fine but wouldn't fire. Seemed to only happen when it was colder out and battery was spinning slow so thought a replacement was due. Car ran perfect for 2 days including starting right up multiple times in 0-5F weather. When I rechecked connectors roadside they were perfectly tight as I had left them. I had also wire brushed the battery connectors during install to ensure no corrosion or poor connectivity.

Trying to determine a few things;

Could intermittent electrical connectivity have been resolved simply when I jiggled the cables on the battery?
Given that I fiddled with it back there and nowhere near the CEM assuming this means any issue is toward the back of the vehicle not up front? I know I shouldn’t assume.
Are the symptoms described more indicative of CEM or battery connection? Or are they so interrelated that that they are essentially one and the same?

Any feedback appreciated. The prospect of using the car without knowing if I will be left stranded on no notice is a bit daunting. I have the weekend in my garage to try and sort it out.

 

An overdue follow up to this;

While the main + and - were securely fastened to the posts on the battery, there is an auxiliary + that piggybacks off the main and feeds a fused distribution block in the trunk that powers many accessories including ECM. This + wire that piggybacks off the main was loose and intermittently causing power disruption. Once tightened the problem was solved. I suspect it loosened initially when I needed a booster start in extreme cold. In the process of attaching gator clip must have worked it loose.