lens questions for a alpha 100

[user name]

hi all
i have a sony alpha 100
i bought a sigma DG 28-300MM 1:3.5-6.3 lens
when i use it often the auto focus is trying to find focus, so much so i regularily miss out on a photograph
it's done it sice i bought the lens new
i also have a sigma DC 10-20MM 1:4-5.6 lens
this one was also new, and doesn't do the focus search
neither does the original lens that came with the camera
my question, is the 28-300 lens faulty, if it's not is there an equivalent lens that won't find focus when i don't want it to, so i don't loose out on any more photographs

help me please