Chain going "slack" on the 7th gear under load on a new mountain bike? I recently bought a new Mountain Bike which works pretty well, aside from the fact that I have a problem with it which creates a pretty damn large safety hazard, not to mention creates a massive headache for me.

I'll be in the 7th gear, and when I start to stand up/put a lot of pressure on the pedals - I'm not really sure how to explain this - it feels like my chain slacks for a moment, and my pedals kind of jerk and then everything goes back to normal for a moment, before going back to slacking again.

This ONLY happens in the 7th gear. It used to happen sometimes in the 6th gear, but I messed around with my H/L screws and barrel adjuster and now it only happens in the 7th gear. I'm not sure what is causing the problem.

Does anyone have some advice for me? I've tried adjusting my barrel adjuster over and over again and I can't seem to narrow it down to being that.

Could it be a problem with the H/L screws?

I REALLY need to figure it out, because right now I pretty much can't use my 7th gear because I'm afraid one of these bad "jerks" will cause me to crash, as I've come close to doing it before.
Does the bike have multiple chainrings? Make sure you aren't cross-shifting- that is, shifting into a gear that is too high/low for the chainring that the chain is sitting on. This can cause a problem like you've described- chain rubbing on front derailleur and constantly attempting to shift to put the chain into a more linear position.