Imac 2.8, 10.7.5 freezes after running then screen goes gray

Imac 2.8ghz core 2(early 2008) 24 inch
10.7.5
2 GB ram
320 gb hard drive- 298 gb free

Hello
computer was recently wiped clean and installed with 10.7.5 from previous owner.
Imac freezes after various time. Most times screen turns gray and computer is frozen. Less frequently computer freezes but screen is still visible.
I have check disk utilities, onyx and disk warrior. Have also booted up from recovery mode and everything seems fine.
Computer freezes with free ram available as it has froze several times while just running Chrome with 1 tab opened.
The lcd screen has some dark spots on the edges. Other then the freezes it seems to run fine.
I have apple keyboard and mouse plugged into usb ports with nothing else attached.

I would love to get some advice on this.

Thank You
Carolyn

G'day and welcome to the forums.

Carolyn who installed Lion OS X.7.5, you or the previous owner? If the previous owner would suggest you purchasing Lion from the link below, using Lion DiskMaker to make a bootable USB thumb drive, boot from that and format the hard drive and do a clean install and update the system after that.

Of course, backup first to an external drive. The iMac is approaching seven years of age and may have had quite a heavy work load. 2GB of memory simply will not hack it anymore. Consider increasing. OWC and Crucial are recommended memory suppliers. Your iMac can handle 6GB of memory.


http://store.apple.com/us/product/D6106Z/A/os-x-lion

Previous owner installed after a hard drive clean wipe.
Currently I have added nothing to computer that needs a back up.
I did order 4 GB ram, when researching it said 4GB was maximum. Is that correct?
I have Apple install disk for Snow Leopard but not for Lion. Perhaps going back to 10.6?

I just needed a solid Mac to work on spreadsheets

Carolyn

Sounds like it may be overheating and/or the main board is failing.

Take it to a Genius Bar if you can and get them to run diagnostics on it.

Peter