Chain Slippage on MTB
My first thought that with a new drive train this would be either:


A bent derailleur hanger
The rear shifter cable has an issue


As you have had the hanger replaced recently I would tend to think that it was likely that it is a cable issue. 

The first thing I would do is inspect the cable and make sure it is moving freely in the housing and there is no fraying or bends and that the end caps are all seated well in the cable guides. The more likely thought (and timing for this would also seem right) is that the cable has stretched and this is the cause of the slippage under pressure. There are barrel adjusters on your shifters which allow you to fine tune the indexing between the cogs (the H and L screws set the outer limits). There are many different ways to do this but Park Tools provide a helpful guide here.

My assumption (without seeing it) is that a new RD would not need to replaced so soon unless something was visibly broken or bent beyond repair.