Check the settings of your collision object; depth should be less than half the offset, or you'll get cloth verts popping into and out of the collision surface. I start with 0.2 for depth, 0.4 for offset, and fiddle from there.
Also check your values are all in 'sane' ranges; if you do things like set your stretch resistance too high ( > 3000) or too low ( < 10), the solver will become unstable and give crap results. Setting any values to 0 will also break things, unless you really know what you're doing (not that I do...).