I had the SAME exact problem...I had directv come out and take a look,...and they couldn't figure it out...
I also wondered if it was some branches that looked to maybe partially obstruct my dish, but it didn't explain why on a perfectly still day, my signal on 119 would go anywhere from 0 - 85% I tried alot of things, and then finally I said what the heck on a particularly bad day when I had 0 signal strength, and started moving some branches around,...I found one that I tethered down a bit causing my signal to spike back up to 85% and its never fluctuated since...
its strange because it made no sense that the signal would fluctuate the way it did, even when the branch wasn't moving at all...even the directv folks didn't quite understand it...plus, none of my other satellites (101, 110) were effected either...but for some reason, it fixed the problem
long story short, don't underestimate what a seemingly harmless branch might do to your signal...see if you can tether some wire around it and pull it in such a way as to improve your signal...either that or cut it back...