really strange problem happening since 2005

greetings,
I have a 2005 G5 apple imac. since the day i bought it, its done a very strange freezing type thing. bascally at any random time it willl just freeze and all this text that looks like DOS code will come up on t he screen. the only thing that brings my computer back to life is a hard reset. ive brought it into the mac stores and since it doesnt do it when it is there, they dont do anything to fix it. im attaching an image of what the screen displays when this happens. any help would be great. thanks.

You've been putting up with this for 7 years?
Maybe it's time to treat yourself to a new Mac.

I do. i have a 2007 macbook pro. this is a secondary computer but id like to finally address the issue so i can use it as backup or sell. thank you.

Is the message on screen always the same? Have you noticed what it says? Perhaps that will give us a clue.