Chain going "slack" on the 7th gear under load on a new mountain bike?

[JohnHWinston]

Hello Cycling Forums,
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 hazzard, not to mention creates a massive headache for me.

I'll be in the 7th gear, and when I start to stand up/put alot of preassure 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 becuase I'm afraid one of these bad "jerks" will cause me to crash, as I've come close to doing it before.

 

[alfeng]

What model rear derailleur does your bike have?

Which rear Cog are you referring to as the 7th?

How many teeth does it have?

How many Cogs does your rear wheel have?
​

Does the problem occur on BOTH pavement & dirt-or-gravel OR only on one of the two types of surfaces?

​