I've been working on a clarinet sonata recently - I've completed the first and second movements and I'm really happy with them. Now comes the hard part: the third movement. Whenever I write multi-movement pieces, the third movement always gives me trouble because it should be fast (but not the same tempo as the fist movement) and stylistically related (but not too similar) and pick up the momentum of the previous movements. It's a tricky path to walk.
Whenever I start one of these things, I really need to use Stravinsky's trick and write the finale first. A lot of energy goes into the first thing I write, so that would make a lot of sense.
But I'm taking my time with this sonata because I want it to be perfect. I'd rather take a while to work out a third movement that I'm really happy with than just crank out any old something to complete the piece.