Balloon-like joint system?

I've been trying for far too long now to get a balloon-like effect using joints. I've tried every single kind and they all suck.

Pretty much, the balloon is a very light rigidbody that has an upwards force applied every FixedUpdate. I try to connect that to another rigidbody in order to apply a bit of upwards force to the system. Add enough balloons and it should float. Think World of Goo for the effect I'm trying to achieve.

Unfortunately, the balloon goes crazy and erratically flies left and right all over the place. I've tried countless different settings and I can't seem to get anything that works - no matter what, the balloon flies back and forth in a crappy-looking way.

It's looking like I'll need to code my own system for this, and slerp the balloon position or something while applying force to the linked object. It really seems like the physics system should be able to handle this case, though.

Any ideas?

I added a constant force to the balloon, and attached it to a cube with a configurable joint with X-, Y- and Z motion set to Locked. That is as good as it is going to get I believe.