black screen shows and fan comes on then computer screen freezes

17” iMac insight G5, Model identifier: PowerMac12,1, Processor Name: PowerPC G5 (3.1), Processor Speed: 1.9 GHz, Memory: 1.5 GB, Model: Ma063LL/a

Once I turn on my iMac, the Apple logo is slow to appear. Then, the screen turns blue for about 10 to 15 minutes before the desktop picture and icons become visible.

If I do nothing but leave the computer running, the screen turns black and the fan turns on. It’s frozen in place!

When I move the mouse or hit the space bar on the keyboard to see the desktop and begin work, the computer screen remains black and the fan continues to run loudly.

Then, I have to unplug the computer, wait about 5 minutes and plug it back in to reboot it.

When this first happened, I thought it was a bad hard drive, so I replace the 160 GB hard drive with a 500 GB hard drive a week ago, and still the problem persists.

Any suggestions and quick fixes?