Faulty Mobo or DC jack?

I was recently given a laptop with an unknown history - all I was told was that everything runs fine when the battery is charged from a different computer, but otherwise doesn't charge. My experience matched that. Reinserting and playing with the power cord I would eventually get the plugged-in light to illuminate, and was then able to boot and all seemed well. However after that first time charging it, I was never again able to find the sweet spot.

I had another charger with the same voltage rating and a higher amp rating (higher amps is fine since the laptop only pulls the current it needs) and the same connection size. Immediately when I plugged it in the plugged-in light illuminated once more, and stayed illuminated for hours. Then, again, I lost the sweet spot and eventually refound it some time later. Finally I lost it again, and have never since found it again.

To everyone reading you've probably came to the same conclusion - that the DC jack has a poor weld and isn't delivering power. Well I have one problem with that theory. I've taken a multimeter around the mobo and the board IS receiving power. In fact I can move and manipulate the power cord all I like and the board never looses power.

Is this normal for a faulty/poorly soldered DC jack? Does it sound more like a power controller or blown capacitor problem?