Occasionally very slow charging

My macbook had a bad battery for a while (working at about 45% capacity), and a few weeks ago the machine just blacked out and wouldn't boot up - even with the charger plugged in. I gave it to a repair service with experience with Macbooks, and they replaced the battery. But then when I needed to charge the laptop, charging would sometimes be really slow, saying it could take anywhere between 15-20 hours with 4-5 Watts (according to coconutBattery)

I took this back to the technician and he said it could possibly be the battery he replaced, and he put in a temporary battery in the meantime while he tested the new battery so I could still use my laptop.

But the problem persisted. Occasionally, with a lot of patience and some luck, the machine would charge properly and at the proper full wattage after disconnecting and reconnecting the charger a few times.

I've tried an SMC reset and using a different charger, to no avail.

I'm not sure if it could be a faulty MagSafe DC port or if its something to do with the logic board.

Any help would be really appreciated :)