2010 imac 27" - 2nd monitor erratic?

[imagist]

2010 Imac 27" + second monitor using DVI with genuine Apple Mini DisplayPort to DVI cable - at boot up (everything from cold) - sometimes 2nd monitor works, sometimes just fine colored blizzard.
Sometimes (mostly) if not working properly (fine colored blizzard) can be cured by turning mirror display on and off again, but sometimes it can't.
Can sometimes be cured by reboot - sometimes not.
Occasionally boots up and works fine from start.

When it works it stays working indefinitely until reboot.

imac 27" display res = 2560 x 1440
2nd monitor Liyama Prolite B2409HDS display 1080P (= 1920 x 1080)
iMac11,1
Intel Core i7
2.8 GHz
8 GB

ATI Radeon HD 4850
VRAM (Total): 512 MB

I thought I'd be leaving this kind of erratic behavior behind when I migrated from the house of cards with Windows

Any suggestions please?

thanks.

 

[stroked]

I had similar problems with my second monitor. It was a defective mini dvi to vga connector.

 

[imagist]

It seems to be the Liyama Prolite B2409HDS not locking in initially for some reason. Turning power off and on again to the Liyama makes it work every time.