Q: panic / Kernet Trap / debugger called ?

On starting my iMac O S X 10.6.8 I get a transparent shroud drop down over the page with a message "you need to restart your computer by holding the power button down untill it turns off and restart." Also in the top LH corner of the screen is a fault message that includes the words 'panic' 'Kernet Trap'  'debugger called' together with a table of combined letters and figures.

I asked the local Apple store for advice and followed their instructions of (a) remove and replace the memory boards and (b) press down on P/ R/ alt/ cmd at the same time while switching on the computer. Niether of these have solved the problem.

Sometimes the computer stays on for a period, thus I am able to get this message out, but mainly it goes down immediately it had booted up and sometimes even during the boot up process.

Can anyone please help solve this problem?

Thanks MalcolmD