K 50 focus problem?

I am currently holidaying with a 3 month old K50, which has about 2000 shutter activations, and it has intermittently started to refuse to focus. I am using a Sigma 17-70 lens. There seems to be no pattern . It occurs in good light, with no particular subject. I have tried different AF patterns, modes ie AFS and AFC, and the focus motor seems to be working. I have my older Kx with me and some lenses such as the WR 18-55mm that came with the K50, so I propose to work some combinations to see if it is lens or camera related and I can isolate the cause but would welcome any help. I have also checked the sensor ( no dust spot) and activated the dust removal/shake function.

is this known problem or does anyone have any thoughts?

I also have a 17-70mm Sigma lens which won't focus properly towards the long end when the aperture is wide, so I stop down with ISO set to Auto and then it works fine.
Happens with both cameras, the K-50 and my K-01.
No other lens I have behaves this way.

Thanks Steve

I will change lenses and see how that goes. It is strange though as I have had that Sigma for a year or so on my KX. No problems but it was not used heavily at home. here in France it has done several thousand shots. Who knows?

I should mention that mine was bought 2nd hand, so possibly it's a fault that the seller failed to mention or it's an inherent issue with the lens. Either way I work around it