Tamron 17-50 problem

Having some problems with a Tamron 17-50 2.8. The AF works fine from 17-34mm but 35-50mm it just hunts but never locks. Doesn't matter what the distance or the light.
Does anyone have any ideas what the problem maybe? Or how much Tamron will charge to fix it?

Desperate to get it working.

I have never had that problem with mine Russ, but I will have a go tomorrow and see if I can make it happen.

Many thanks Andrew, that'd be great. It's weird that only part of the zoom range is affected. I sort of have two lenses now. A 17-34 2.8 AF and a 35-50 manual zoom.
Sent in a service request to Tamron, see what they say.