Cannot get to the Apple Hardware test in mountain lion
I'm having lots of strange hardware issues with my iMac 27. The system keeps crashing, so I decided that I wanted to check the system with the AHT, Apple hardware test. Following the instructions (to press cmd-D, or just D) after a system re-boot, (waiting for the start-up sound), just doens't do the trick. The computer boots up normally, it seems to take much longer time. but in the end I just get the log-on screen.
Any ideas how get into the hardware test?
iMac, OS X Mountain Lion (10.8.2)