Chain Rattle & Gear Jumping

Trailrash

Hi Peeps!

I'm a newby to forums and getting to grips with "best bike set-up" issues.
I bought a 2008 Commencal Meta 5.5.2 and it's been great, had it serviced a couple of weeks ago which involved relubes, checks and new cables all round. The bike ran smoothly for a few rides but is starting to jump gears a bit again. I understand the cables wear-in a bit and stretch etc. so I've tensioned them up a bit but I still get some gears jumping, even though the indexing seems fine, and THAT chain rattle when you're ragging down some lumpy slopes! Does that mean the chain is too long, as they did test the chain, chainrings & chain set for wear at the last service, or IS it in fact worn, or could it have something to do with the rear shock set-up i.e. have I got it set with too much sag causing too much slack in the chain when it's working really hard? :?