The gears on my mountain bike keep slipping?

[Dexter49]

I bought a mountain bike a while back to get into shape in the nicer weathers but whenever I ride it the gears seem to slip randomly. Its a grip shift and even when Im nowhere near the grip the gears slip, as you can guess, it makes for an awkward ride as the pedals jolt when this happens.

At first I thought it could be because Im quite a heavy guy but Im not so sure. I just want to get it sorted as I'm sure its not doing the bike any good. Anyone know of any possible causes?

 

[maydog]

I assume that the slippage is in the rear?

The first thing to check is the derailleur trim. In a gear, the jockey wheels should line up with the cog in a cassette. You can turn the barrell adjuster at the derailleur to get a good alignment.