Wow, I thought I was just going through an odd patch, but it sounds like we're having the same problem. I don't know about you, but I don't even find it's a conscious thing. I don't practice voices like I should, but I find it naturally drifts upwards when I'm in comfortable company and I can feel like myself. It can suddenly dip without my thinking about it though, and then sore throats creep in. I think it's just a small slip-up which is making me nervous, and that nervousness makes my voice all the shakier. The only solution I've found is shutting up for a bit, which isn't much fun. ;P