imac does not detect external vga display

Please help with connecting external vga display to an imac 21.5'', late 2012, OS X 10.8.5 via Mini DisplayPort to VGA adapter.

I have connected an external monitor (Fujitsu Siemens) to the thunderbolt port of the imac using a Mini DisplayPort to VGA adapter. The imac does not detect the external monitor. I have followed all instructions and suggestions I could find at Apple Support Communities without success.

The monitor is in perfectly good working order as it has no problem connecting and performing flawlessly with a dell windows laptop. The imac, connected via the Mini DisplayPort to VGA adapter to the D-Sub connector of a Samsung smart TV, also worked perfectly.

To add insult to injury, occasionally, when restarting the iMac with the external vga display connected and powered on, the iMac screen starts intermittently turning blue for a brief second or so and resuming back to normal. The frequency of 'blueing' is once in 2-3 seconds. This particular rather disturbing problem, luckily ceases when the external display is switched off.

Any advice and help with resolving the above connection problems will be gratefully appreciated.

Thank you

ghogoh