iMac boot only after NVRAM

Hi ,

I have an iMac who at the very beginning had a graphics card problem (black screen at startup). Not having wanted to buy a new graphic card directly, I took a card also from an iMac A1311 but less powerful; HD 6770M to HD 4670. Only for the tests.

After installing the card, the iMac restarted but stay on a blank page and cut itself. The LED code changed from "Blink green green" to "blink blink blink green". Knowing that I didn’t pull up the screen and that the display is external.

I tried my tests with a screen , same result.

By doing tests, the iMac is restarting after a NVRAM ($@$* + cmd + p + r ) and magic the iMac started on the OS-X without problem.

The system runs flawlessly with 1080p videos that play continuously for hours.

But when I try for example an "Apple Hardware test", the iMac systematically turns off at the quarter of the load of the extended test (memory test), I obviously change the RAM, with no result.

After if i want restart this imac i need to reset NVRAM , if i unplug imac NVRAM reset , if i shutdown normally the imac NVRAM reset , only reboot work well.

So do you have informations for me ?

Thank You