My 30-year-old 28-70 G has been in the shop three times over the years, twice in the last 18 months. I was having problems with the aperture not closing properly.
Today it works well on my a900, but not on my a99... Sony maintenance tells me the mechanism that drives aperture on the a900 body is more "robust" - another way of saying powerful - than on the a99, which is more geared toward SSM. With age, the lens is becoming less reliable and needs more force to activate it. The control spring in it has been slightly loosened and I had it cleaned for excess grease that could slow it. In warm weather it works fine, but when its 5° Celsius or less, it becomes inconsistent. And of course one can't find spare parts anymore.
So I am starting to look into replacing it, particularly as I will probably get an a99ii next year. I was told to avoid the Tamron equivalent because of sharpness issues at 70mm. What other excellent zoom in that range do folks recommend - should I get the Zeis 24-70 ? Money is an issue - but not decisive...