The jumping is either chain friction (ie the links are not moving smoothly against each other) or links catching on teeth.
Fagan.co.za has a useful article on chain suck.
To summarise his points, the most common causes of chain suck are:
- Mud causing extra friction
- New chain with a worn chain ring
- Worn chain with a worn chain ring
- Worn chain with a new chain ring
Because the chain loads up unevenly on teeth on the bottom of the chain ring you get high friction, and the chain won't disengage properly from the teeth.
First up - clean your chain and lubricate it properly.
If the problem continues, the simplest fix is to replace the chain ring, but you could also get your teeth reprofiled to reduce the friction.