Working with chocolate in small batches (by hand) is like shooting at a moving target; you always have to be making minor adjustments in order to keep the viscosity consistent.
Here are a couple of hints that may help:
1. White chocolate takes longer for the beta crystals to propogate appropriately, as there are more "non-cocoa" particles in it. You need to have more patience.
2. When you see your chocolate start to get thicker than you would ideally like, hit it for a moment or two with a blow dryer, and stir like crazy.
3. Don't wait until the chocolate is unbearably thick (over tempered). Make constant adjustments as you go, but keep in mind that you need to stir it very well.
I'd type more, but I'm on hour 16 of today's version of Christmas insanity and I desperately need food. Hope that helps.
...PS.... When you think you've stirred enough, Well... You haven't.