My weirdo Z2 not charging using stock charger topic

I have a weird problem with my Z2..

few days back it stopped charging through the usb port and couldn't be detected by my PC, I did try different chargers and cables.. but it did charge through the dock..
Took it to Sony service center and they declared the mother board needed to be changed and it was expensive.. almost same as the price of a new z2.. the guy there advised me to take it to any private repairing shop, changing only the usb port would be cheaper.. so today I took it to a repair shop.. but to my surprise the technician there said my phone was charging fine..
I was not sure but it definitely was charging... so brought it back home.. and... not charging again... tried all chargers that I had... tried connecting it to PC, nothing ! it did not go to flash mode even.. but did go to download mode..

and then I tried a 2.1A charger.. and there it was charging as if it had no problems ever !

So its charging with a 2.1 A charger but not through the genuine 1.5 A charger.. though it does still charge through the dock fine... and it still can't be detected by the PC..

Any ideas.. ??
I am still guessing its hardware but hoping its not...