chain jumping when puilling away in a higher gear I replaced the rear cog as the chain I had snapped and I thought it had taken some teeth off. Now when I pull away in 1-3 gears it's ok, but if I am in gears 4-7 and I try to pull away ( say I have stopped at lights and not had chance to change back up to 1st) then the chain slips and bounces off the cog its on. If I change up through the gears when I am already riding it isn't a problem, just seems to happen if I put any tension on the chain in the higher gears when I have stopped and start pulling away again. Could this be a cog problem? set up or chain problem? Its done it for a while now and I have put up with it, but I am overhauling the bike and would like to have a heads up as to what I should be looking at to fix/ replace.
When you replace the cluster (rear cogs) you almost always need to replace the chain too because they wear together.  If you put an old chain on a new cluster, you often experience that kind of because the length* of the links in the chain is slightly longer than the spaces between the teeth on a new cluster.

*Note the links don't actually get longer, it's just that the rollers wear out and allow a little play and the links sit a little further apart.  This is known as "stretch" though it's different to the way something like elastic stretches.