Chain issue

[devo]

Hello, having this very irritating issue with my bike. It is an 18 speed mountain bike, and just recently while pedaling the chain will skip forcing the pedals to randomly be forced around. I have tried lubricants and cleaning the chain, also re-aligned any sprockets.
Thanks in advance

[Joe_W]

1) Chain / sprockets / chainwheel can be worn. Check for chain "stretch" and worn cogs.
http://bicycletutor.com/chain-wear/
2) setup can be off, but you wrote that you already checked it.

[Joe_W]

Oh, I forgot: it could also just be a stiff link. Check for that by slowly pedalling backwards and closely observing how the chain runs through the rear derailleur.