Shifting automatically/coming out of gear

[Driver]

Hi,
My bike never seems to stay on the 6th(out of 7) gear on my rear derailleur. Whenever I shift into this gear, it usually stays there for maybe five or ten seconds, and always slips out of the gear onto the 7th gear. Is there anything I can do to fix this?
Thanks

[Painkiller]

what type shifters do you have? If you have more than one front ring does it do it in all of them?
Is there cable slack when in number 7? I would say not adjusted correctly, bad shifter or broken or damaged cog. look in this order