M15x video card blocking POST test after memory upgrade

M15X working fine with 3GB RAM. Changing to 16GB results in 3 beeps and the LED code says a problem with the video card (GTX460M) is blocking the POST test. Laptop stops with the LEDs flashing and nothing else, no screen image. Replacng the 3GB sees it all work well. Does anyone have any suggestions why the video card would react badly to a massive expansion in RAM please? Laptop is running Widows 7 Home Premium with Q740 processor.