Chain slip. help!

[Melonfish]

I've done 1200 miles on my dawes Mojave 2011 since the end of may.
recently i've been getting chain slip on the cassette when in my normal cycling gear.

its a 7 speed cassette with 3 cogs. i usually cycle in the middle cog and use either the 6th (second smallest) or 5th gear (third smallest) gears, where i am is quite flat or if i'm motoring i'll use the 7th gear quite happily.

now i've been noticing that the chain slips when in the 6th gear middle cog. it doesn't do it if im in the big cog or the little, its knocked my confidence in the bike because i'll be on a busy roundabout and the chain is slipping, or i'll stand up to get over a smallish hill and the chain will slip.

i've adjusted the barrel adjuster and that appeared to work for about 5 mins, now i'm stuck with some chain slip and a finicky downchange.

what should i do with this? i really need advice, i've never indexed a derailler before does it need this or is it something more serious?
also, no visible signs of chipped/cracked cogs on the cassette, i checked thoroughly.
thanks
pete

 

[fossyant]

Worn chain and or cassette.

Indexing - you'll hear the chain tinkle more and it won't shift cleanly.

Worn cassette and chain can does result in chain slipping. What mostly happens is you then put a new chain on and the slipping is worse as the sprockets are worn.

As you are only using 2 of the sprockets mainly, and these are the ones that are slipping, I would buy a new chain and cassette.

 

[Melonfish]

actually i think you ahve it spot on, testing it earlier, in any chainset the 6th gear slips. looks like i need a new cassette.

picked a replacement online, also replacement chain (best to replace both at once i've read) and i'll swap em shortly.
i suppose its one way to get out of cleaning your chainset

i guess it just shows how much i use this gear, in both my middle cog and large outer cog, when i get up to speed i sit in 6th and spin happily.
ho hum, fettlage here i come.
ooh that rhymed.
pete

 

[HLaB]

Before you go to any expense, I'd clean and lube the mech and cables, the cables may be snagging at around 6th gear then suddenly releasing and jumping over it. Unless you have been very lax on cleaning and have ridden in very harsh condition I wouldn't expect it to start slipping at 1,200 miles. However you've already bought the chain/cassette, if it still jumps then replace the cable inner and outer and as to indexing this is the clearest vid I've found so far, for me it removed the mystery of the limit screws (basically you won't need to touch them if they are already set right).
[media]
]View: http://www.youtube.com/watch?v=SkzvfCaIbyQ&list=FLMkVidrNbLVxmrMOyP5uL7A&index=1&feature=plpp_video[/media]

 

[Melonfish]

I had a look in daylight at lunch and you can visibly see the difference between the width of the teeth on the 6th and 7th cogs. its quite alarming.
i do ride in harsh conditions, daily at that and i don't clean it as often as it deserves, however the visible difference between the teeth really shocked me.
tbh its not a major expense to replace and its one of those things i could do with learning.
regarding limiter screws, they're all set so i shan't be messing with em aye.
pete