That very much sounds like a soldering issue, but it could also be an issue of a loose component — for example a chip not fully seated in a socket. To start debugging, it would be helpful to try to reason from what the exact behavior is. For example if the display is going off but it’s still keeping time, then it’s probably something in the display drivers, rather than something to do with the power. If it’s fully losing power, then start looking at the power jack, and see if (for example) you can find a point that you can wiggle to cause it to lose power. If you can identify where the problem is coming from, then perhaps there’s something not fully seated, or something that needs to be resoldered.
Also visually check for loose or missing solder joints of course– these are by far the most likely cause of issues like these.
