In mid January, 2014, I had purchased MyBike SS form Decathlon Ahmedabad. The bike is truly a good bike, it runs smoothly and quite reliable for the price that I bought it at, have barely spent a rupee on parts for this bike within its first 1200 Kms.
But, I have been having one issue with this bike, the freewheel has been giving me problems, although I have been getting it exchanged under warranty. Currently I am using my 5th freewheel. Its not that the freewheel lacks quality, its really smooth while riding, thats the main thing about this bike, it runs smoothly.
But why is it that the local quality/brand of freewheel lasts much longer (except the part that they don't run that smooth)?
With my first and second freewheel (which I exchanged because it made cracking like sounds at random intervals while riding), I was told by Decathlon later on (when I went to exchange my 3rd freewheel) that it occurred because the manufacturer put less lubrication/grease in the mechanism (I had given the recording of that noise to Decathlon).
The third one, the Ratchet mechanism stopped working and had to use it as a fixed gear (fixie).
They told me that my 4th one (freewheel) was given with ample amount of grease/lube. But stopped working after ~2 months. One day while casually riding on a flat road, the pedals/chain started moving independently of the wheel, I was pedaling but the wheel wasn't moving.
From your description of the problem (very accurate), it's clear that the freewheel of the MyBike has a serious issue. I guess it is not designed for extensive riding, and perhaps only suitable for light recreation.
When you ride a fixed gear bike and want to climb a gradient or quickly speed up after a stop, you normally pedal standing up, and the freewheel cannot take this extra load on it.
If you like to retain this bike, a good option is to buy a TI brand freewheel from Hercules dealer and get it fixed by them. This will last quite a while.
The alternative is to exchange with a 18/21 speed bike where you don't need to put extra pressure on the pedals while climbing or pick up : you need to just downshift the gear and keep pedaling.
I can't see any other solution right now :)