focusing issue... need advice pls

Hi,

my pentax K-5 with 18-55 kit lens is front focusing in incandescent light and in low light as i experienced.

i did AF fine adjustment to the end, it helped, but I checked it is still front focusing for instance around 20 cm with 35mm focal length...

in day light it seems okay without AF fine adjustment...

my question is that it is due to Camera of it is due to lens?

do you think will it be okay if I change my lens? i am plannig to upgrade my lens anyway...
otherwise i have to eiher return me camera or send it to get fixed?

could you pls advice?

thanks & regards,
murat