Sigma 150-500mm focusing issue

Had the Sigma 150-500mm for a few months now and it seems to have possibly developed an AF fault. It focuses pretty quickly and accurately up to about 20 metres but anything distant hunts badly. This happens in good contrast situations and on simple subjects like a sign or a distant house. It just goes backwards and forward either side with a sort of clicking noise. In the same conditions the DAL 55-300mm snaps on focus easily. It does this for all AF focus settings and with and without all OS S/R combinations. Every now and again it seems to work normally. I don't seem to remember this problem when I first got it. Any suggestions?

Is it still under warranty :

If so it may well be best sending it off to be checked out

Yes it is. Will try and get into SRS where I got it from and see what they think.

I would just ring Sigma armed with your serial number. I had a problem on my 120-400 that was down to a recall that my particular example had missed.

I spoke to them Tuesday, Posted it on Wednesday with them saying around a week but I had it back on the following Tuesday all repaired and ready to go.

Excellent service.

Yes, just a quick update. FIxed by Sigma via SRS with little stress and it is now a different and effective beast altogether.