Auto Transmissions are quite difficult to diagnose, and I know little of Auto Tranmissions, other than general operation. But, that said, it does sound as iff something is going wrong, perphaps with the clutch packs. Since it is under warranty, you simply must make sure the problem is detailed and formally entered into the service record as a complaint.
Now, assuming you are going to hit 30,000 before the warranty is over (or perhaps even if it isn't, as you are getting close), and need the fluid changed, you might want to go to the extra expense of having the tranmission filter changed. Nowadays most places do not remove the tranmission pan or replace the filter, they just use a costly, specialized machine to take out all of the old fluid and replace it with new fluid, never removing the pan or changing the trans filter. As less than half of the fluid gets changed using the "old" method of pan removal, there is some sense to the newer method. But if a transmission is having problems, it often can be seen if the pan is removed for filter replacement, as they often is a lot of crap and shavings in the pan/filter.
Insisting that they change the trans filter, as well as change the fluid, will cost you a few extra bucks, but if they are an honest dealership, a pending problem might be found from debris and/or excessive metal shavings on the magnet located inside the pan (I am assuming the Focus has such a magnet, as most auto transmissions do).
If it was my car, I would spend the bucks to have the pan removed and filter replaced simply to ensure this inspection for debris/metal particles takes place before the warranty runs out.
A really good dealership would test the transmission and, if they saw the same symptoms as you, take off the pan for an inspection anyway.
