DH writes: "Help - pressure! I'm not sure I've got a lot more to add than what Katisha and Mrs Badger have said. Really hard to do this without looking at the notation you're using!
For me, it's all about getting an outline, then filling in the gaps. I would always recommend getting your cadences in first, then figure out a strong chord to start each phrase on. Then you can start filling in the gaps in the middle of each phrase.
I'm guessing you're at a stage where you've got all four parts in at the moment? If so, what I'd do at this stage if I had a jumpy bass line would be to go back to thinking about the chorale in just two parts - the treble and the bass. If you start by looking at your treble line, and thinking that each note needs a bass note that is either 1) the same, 2) a third below, 3) a fifth below or 4) a sixth below, you'll realise you don't have too many options - quite a liberating feeling. But you'll also (hopefully!) be able to find a path for your bass line which is a bit smoother. (By the way, by a third, fifth etc. below I mean 8ve plus a third fifth etc.)
Once you've done that, you can start to think about how to fill in the gaps - i.e. the alto and tenor parts. I personally wouldn't fret too much about whether you're using a IIb7 or a VI or whatever - as long as the starts and ends of phrases are strong, harmonies in the middle generally look after themselves.
I tend to find that you get jumpy bass lines when you're determined to use a particular chord - i.e. thinking vertically - rather than making sure each line works well with the others - i.e. thinking horizontally.
Good luck with it - and do say if any of this is unclear!"