Grammar underlining (green dashed line) that flickers ?

I have a grammar underlining (green dashed line) that flickers under one character in a word as I keep on writing further down in the text, several sentences away from the location of the flicker. What's strange is that the minute I stop writing it disappears. I can, however, make it appear by clicking on the space bar for example. When I click it (Ctrl + mouse left button) I get the opportunity to look it up, the single letter och sometimes group of letter that the flicker appears under. It has at times occurred under a complete word for no reason ( grammatically ) what so ever. It's not catastrophic in any way just very annoying...

Thanks!