"Sometimes the laptop would just turn off while plugged and to make it work again (random), I would have to unplug and replug the ac adapter to get the unit to boot up again"

This clearly indicates that one or more legs of the socket for ac adapter
came unsoldered from the motherboard. You could try to solder it again
but it seems to be not very trivial. My D4750 is 2.5 years old; the battery
died 3--4 month ago and the socket came unsoldered completely.
I've tried to solder it again (without removing the proc) but failed
to get stable soldering. The ad hoc solution I use is to insert
a piece of hard eraser between notebook's cover and
the socket so that the socket becomes tightly pressed to the motherboard.
I have now no battery at all, but this solution works for me more or less satisfactorily.
Sometimes the contact gets lost and I am to readjust the contact and
to restart the notebook, but this happens rarely enough.