Your lens may have been serviced, but was it calibrated?
You can check for focusing errors yourself. Look up on the net, do the test. If it is due to focusing error, print the test result and send it back it again for calibration.
I sent my tamron 28-75 back to Japan for calibration TWICE before it was acceptable at f/2.8 (took more than 3 months in total) ... will post some pics if I have time ;p
I sent my tamron 28-75 back to Japan for calibration TWICE before it was acceptable at f/2.8 (took more than 3 months in total) ... will post some pics if I have time ;p