Not too sure how to tackle this but...usually when the gears jump is it because the chain is nackered?? How do I check??
more than likely it'll be your rear mech that isn't set up right, may need a little tinkering. if your not sure how to do this, either goole it or take it to the shop, no point f**king your kit up by bodging it yourself
Best get shop to check if chain is stretched unless you know someone who works on their own bikes. Its a very simple tool to measure if its stretched. Doubt you'll get charged just for checking.
You probably just need your rear mech adjusted and maybe the gear cables replaced.
I had the same problem with my bike and this sorted it out. Shouldn't cost much to get done at a good bike shop.
Testing the chain for stretching is also a good idea. I think there was an artical in 220 an issue or 2 ago about now to do this with a normal ruler.
You can easily measure for chain stretch yourself:
But it is more likely to be due to adjustment of the rear mech. This is easy enough to do yourself and again Sheldon knows all:
This video is rather good for explaining how to check the indexing of rear derailleurs.
could be one of a thew reasons.
1. your rear mech needs setting up
2. your gear cable is streched and need replacing
3. your chain is streched and needs replacing
you best bett is to take it to you local bike shop and get them do/check the above.I would advice you to get the gear cable replaced if and only if streched and get your local bike shop to do it and they will set up the gears for you over wise just do number 1