Imac 24" (mid 2007) startup problems - blue screen with pixels, safe mode fine

Recently I replaced my 2007 iMac 24" HD with a 2 TB drive. Prior the iMac would randomly freeze especially when using Safari and a few times the questionmark folder appeared but would still boot-up after second restart luckily managed to backup everything. Ordered a new HD and installd it myself

with great care and attention to the ifixit instruction and videos on Youtube. iMac worked like a dream after fresh installation and no glitches or freezes

A few days after however shortly after startup the iMAc would boot to the Desktop without probs but then get stuck completely, no force quit or mouse movement watsoever only panic button shutdown. After rebooting the screen suddenly displayed strange lines and scrambled pixels (once booted to desktop).

I reset the PRAM (alt+option+P+R), exchanged the PRAM-battery diconnected the powercable and mouse/keyboard, peripherals waited for 20 seconds and restarted...to the same problem. After many attempts the screenimage stays blue with a little row of pixels that can be moved with the mouse on the screen however no image showing. I can reboot in safe mode (shift at startup) and the image shows perfectly sharp with no issues.

At startup also the apple logo would show and the the gray screen which then gets blue and then the misterious pixels would appear.

I am suspecting a faulty Graphics card or some issue from the logic board to the Graphics output. After running AppleHardwareTest all is fine no problems reported. I first scanned the HD booting from my disk as there where some repairs to be done in the Library files which repaired but nothing changed.

Reformatted the HD and installed the original OS on the HD ( Tiger). Still no image displaying, only that row of pixels. I'll attach an image that hopefully will shed some light. Also a big thanks to all the people who have written and shared their problems & knowledge on here before me as this has helped me a lot sofar.