my human iPS cells have a lot of differentiation - (Aug/27/2012 )

My iPS cells keep differentiating spontaneously. I'm on mTeSR/Matrigel and I am able to maintain alot of undifferentiated iPS cells but I always get quite a bit of differentiation. I haven't had too much experience so how much differentiation is normal? What is the best way to passage these? For me the best way is using dispase, manually picking the good colonies, and breaking them down by gently pipetting. But is there a better way? Has anyone tried Invitrogen's EZpassage tool?
Thanks in advance

Colleague did (tried the tool). Said it helped a lot.

Manually picking the good colonies important if you have a high degree of differentiation.

I am wondering if anyone can compose a list of factors that would induce spontaneous differentiation BESIDES errors in manual dissection passaging? Like having unhealthy MEFs, or not changing media frequently enough.. Is there an article somewhere I can read about this?

We are using Pluripro in our lab. The system uses single cell passage for hES cells, so it is very easy to work with and there is no differentiation.so no need to cut round colonies. A lot of labs that are culturing lots of clls are starting to switch over to this.