Flickering on terrain in some areas

HI .

I have a mesh imported on top of terrain , as its lowered down far enough to hide the outer area which doesn't need to be seen ( all flat),yet playing game I see flickering sometimes. Its not a lot, but its def. not going to look good in a game, assuming its not my gpu or something equally odd.

Any idea what might cause this ?

I can provide a picture if needed though I'm not sure it would even show up.

thx
jb