Slight surging on constant throttle

Ok my chain seems to be ok (i think) i just adjusted it after buying the bike as it was quite loose.
Dont think its stretched the sprockets are fine the bike has only done 4000ks any ideas what might be causing this?
Any help or ideas really appreciated!
Graeme
Dont think its stretched the sprockets are fine the bike has only done 4000ks any ideas what might be causing this?
Any help or ideas really appreciated!

Graeme