Text definition not sharp, slightly fuzzy

Imac 24" Mid 2007 running 10.10.2

I have noticed any text on the top toolbar and when right clicking seems just a bit fuzzy as if it is not in full resolution. This has only just started happening over the last few weeks.

Any other text is as I expect in Safari and email.

My Twitter app is fine but again if you right click it seems a touch fuzzy and not in the definition/sharpness I would expect.

Any ideas what the problem is ? Not a major problem but just annoying