United Kingdom - Chain jumping off middle cog. Possible causes?

I have bought a second hand GT Avalanche 3.0. Everything else on the bike is in good condition and works fine, but as soon as I select the middle cog and put any pressure on the pedals, the chain starts to slip and jump from the middle cog. What could be causing this?
As you can probably tell I am new to mountain biking.

Dirty Roger,

Derailleur systems are pretty much the same for road or mountain bikes. Here are some things I would check:
1) Is the chain worn and stretched? Do all of the links twist freely or is one tight and bound.
2) When you sight along the chain from behind the cluster and rear derailleur does the chain follow a straight line. (You are checking to see if the derailleur hanger is bent or misaligned.)
3) Try turning the barrel adjuster on the shift cable a quarter turn tighter. This may also simply cause the chain to jump from the cog next to the middle cog.
4) Are any of the teeth on the middle cog bent or worn?
5) Are all of the mounts for the cable and drive train tight so none slip when tension is applied?

Someone else may be able to give you a better answer.

http://sheldonbrown.com/derailer-adjustment.html

http://www.parktool.com/repair/readhowto.asp?id=75

http://cycle.shimano-eu.com/publish/content/cycle/seh/nl/en/technical_service/faq_s/general_faq_s/how_to_adjust_a_front.html

Did you mean the middle cog on the cluster or on the chainring?

poltergeist!
(srry couldnt help myself)