While pedaling standing up, bike crank gives in as if shifting, what is causing this?
Since it only happens when you stand I would say it is definitely a combination of a worn chain and worn chainring, and nothing to do with shifters or derailleurs.

As a chain wears, it 'stretches', meaning the distance between the pins in the chain grows. This is a result of the side plates wearing into the pin. Once the chain starts to 'stretch', it starts to wear on the teeth of the chainring. If you look at it close you will likely notice that the valley's between the teeth on the chainring are not uniform, and the the tips of teeth are wearing down, compared to a new chain ring. 

When you apply torque to the crank, each tooth of the chainring pushes against the roller on the pins of the chain. On a bike with a rear derailleur, there is a spring which keeps tension on the chain. As the chainring teeth wear out, and you apply a certain level of torque on the cranks, it becomes possible to exceed the tension applied by the derailleur and the chain will simply 'roll' up and over the teeth on the chainring, effectively skipping a tooth. This usually happens very suddenly.

The only right solution is to buy a new chain and new chainring (or chainrings). Putting a new chain on a worn chainring or a worn chain on a new chainring will only accelerate the wear on the new item. The bicycle shop should have sold you both.