JP, I have found the same thing! It is so easy to take bad shots with a long lense!! I think Crunch hit it, they use better glass in long lenses. My wife has a really good short lens, a 12-60 zoom that takes excellent, crisp shots. Course it was over a grand for the lens... often you get what you pay for I guess..
Yeah, I just "downgraded" to an old OM-mount analog 50mm lens, using an adapter ring. I haven't used it much yet, but I bought it for the better glass.