Dell S2240L no 1080p

Help? Please?

I am using a mid 2012 10in air with a mini dvi (thunderbolt) to HDMI connector.

I have a Dell S2240L monitor; it will display 720p fine; 1080p when the image is displayed is perfect, however, the image cycles from black, to image, to snow - it looks as though the S2240 will not accept 1080p output, or the Air will not output 1080p.

So, I've connected the Air to two other HDMI devices, and both are fine. I've connected a blue ray player to the monitor and it's fine.

So, I expect that there is some conflict in setting between the Air and the Dell,

I've tried:

1) DDD/CI Enable/Disable

2) Energy Smart Enable/Disable

3) Auto Brightness

4) Underscan

5) PAL/NTSC (50/60hz)

so far nothing I've tried works.

Help?