Macbook Pro and External Monitor flashing?

I am using a Macbook Pro with an External Dell 1908fp 19" monitor via a Thunderbolt to DVI adaptor all fine.

However the annoying niggle I have is that the monitor which seems fine, keeps flashing intermittently on and off randomly?!?

Would anyone know why this is happening and if its the Mac's fault? I have the MBP open and the Monitor as the 2nd screen at 1440x900 which it seems to have defaulted to, and its very clear on both screens.

Is my monitor on its way out? it seems ok, and is quite new (though it was 2nd hand a couple of years ago) or is this a known Apple issue?

Thanks for any help