Back focusing issues with 120-300 2.8 (1 Viewer)

Been having problems with back focusing with the Sigma 120-300 2.8 lens.
Target point out of focus and capturing 12-18 inches back from this.
has anyone had similar problems with this sigma lens ?
Has anyone had the same problems with the 300-800 lens ?
What experiences have people had with the Sigma service centre with related issues ?

Apologies for the number of questions but as you can imagine I would like a rough idea of what I am in for, in respect of standard of service !

I haven't bought this lens but I read some reports on it by reviewers and buyers complaining of the exact same problem. Sorry. Definitely go to Sigma with this, or the shop if poss.