My iMac g4 1ghz wont boot

My iMac g4 1ghz has a problem that im not sure about when i turn it on i hear the apple noise and then it just stays blank on the screen, other times i run it goes to the apple boot logo then freeze and somtime it completes the boot into the operating system (tiger) but then freezes up i have removed the 3rd party intsalled ram but i still get the same problems

When i turn it on there is always power to my mouse and keyboard but im not sure what is the problem with the machine.

PS the mac has been opened before and had a 120gb IDE hdd inserted but they have been confirmed to support this amount of memory.

Did the problem appear after the RAM and/or HD work? It looks like you already removed the 3rd party RAM. Have you checked to make sure the other RAM chips are properly and securely inserted? Also the same goes for the HD. Are you able to boot using the original installation disk?

I have removed the hdd and the ram and tried different ones from my other imac and the problem continued. when i have inserted the Mac os x disk i can hear the drive working but then it get a text string ending with a darwin kernal dump