Windows 7: Problem-external monitor on ATI 5650

I am using Windows 7 Ultimate x64.

I am trying to connect Hyvision by Megavision MV177 17 LCD Monitor - 8ms, 500:1, SXGA 1280x1024, Black at TigerDirect.com (17" external monitor)
to my Acer Aspire 5740G.
The video card is ATI Mobility GPU
When I restart my computer it shows image right before the login screen.
If it shuts down right after that it would be OK, but there my problem begins :
The external screen refreshes constantly. Going on and off when it pleases, changes resolutions, some times stays stable for over half an hour, but sometimes it doesnt even detect it.
When I hit the "detect" button it shows (sound of hardware plug in) then right after disconnects (sound of plug out).

I have the lastest drivers from the official site, my Windows is up to date as well as my DX.

anyone got any ideas ?