Pedals skip while in 7th cog (highest gear)

[Tricericops]

My buddy and I were tuning up my jenky old mountain bike with some new parts. Everything is back in place but when I'm riding in my top gear the pedals skip about a quarter turn every half to full rotation while climbing/accelerating. Everything seems to be fine in my other working gears and it doesn't do it if i'm cruising or pedaling downhill.

We've been trying to adjust the tension just right because it almost feels like the chain keeps slipping off the end but the last couple hours have made us think otherwise.

Any ideas/suggestions?
thanks.

[nfmisso]

Possibilities:
* chain - stiff link(s)
* chain new, and freewheel old; old stretched chain worn into freewheel
* chain old, and freewheel new: chain stretched
* rear derailleur worn, or not properly lubricated, or spring tired
* chain too long

[RobAR]

* or Hi limit screw not adjusted properly (too far out) and actually allowing the chain to ride out enough to frustrate but not fall off. 1/4 turn might fix it. It may take 2.