Chain Slippage on MTB This is my first time posting on this site so bear with me. I've got a 2009 Fuji Tahoe Comp 29er MTB with upgraded SRAM x.7 components and I'm having a couple issues. It is currently set up as a 1x9 and on gears 2-4 and sometimes 5 I'm having severe slippage when under pressure. Back in June I had pretty much the entire drivetrain replaced (front chain ring, bashguard, rear cassette, and chain) and had it tuned up for the summer. I took a pretty hard fall a month or so ago and bent my wheel a bit. I've since had the wheel re-trued and it's just fine and also had the derailleur hanger replaced because it was bent in the fall. My LBS stated that everything was fixed and I shouldn't have any more slippage. It rode fine on flat ground but as soon as I hit uphill terrain and rocks, I started slipping and it basically ruined my whole ride because the technical trails in my area require a lot of lower gears. Anyways, I was wondering if there was anything I could do to fix this. All my higher gears work just fine and the sprockets that are slipping don't show any wear (not rounded off at all or bent) so I doubt it's the cassette. The chain seems fine as well. The LBS I originally took it to said that I may need to replace the derailleur in the near future, which could very well be the issue, but seeing as though they couldn't fix the problem in the first place and that they don't really have a reputation for top notch work I take their opinion with a grain of salt. Please feel free to let me know if there's anything I can do, short of replacing the derailleur, to resolve this problem. I'm dying to get back on the saddle now that the weather has cooled. I'm also back in school and out of work again so the cheaper the better (can't really afford to spend $120 to replace the derailleur). Thanks for your time!

Little more about what's going on: When I have the bike off the ground and proceed to pedal, when the chain slips it seems to bounce back and forth between the rear cassette and the bottom cog of the derailleur if that makes sense. The gears shift just fine with no problems, it is only when under pressure like going uphill where slippage occurs. I take good care of my stuff and I degrease and re-lubricate the drivetrain after just about every ride so it shouldn't be gummed up too much. I want to avoid taking it back to the LBS unless I absolutely have to, so let me know if you have any suggestions for me or if you have any more in depth questions about the bike and whats going on. Any help whatsoever is greatly appreciated. Thanks in advance!
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.