Q: Does anyone know why my iMac G5 osx10.5.8 has suddenly developed continuous small grids of lines across the screen please? I can supply screenshots.

The image appears to reflect the possibility of graphic processor card issues.

Some models had a history of failure or bad solder connections; others may

have a combination of capacitors & other hardware; eventually needed repair.

https://duckduckgo.com/?q=imac+24-in+2008+graphic+anomalies&t=ffsb

One of the cures for some graphic card issue, was a re-solder done by the

users who took their card out of the computer & baked them in kitchen oven.

This idea worked for some; you may be able to contact an Authorized Apple

Service Provider or see if one of the online parts vendors can answer questions

about availability of replacement refurbished graphic cards, if one yours has

can be removed from the logic board or if it is affixed, or soldered to the board.

An iFixit.com repair guide for your series of iMac (Intel-cpu 2008) 24-inch

may show if that section can be practically removed, if so will show process.

https://www.ifixit.com/Device/iMac_Intel_24%22

The computer detail in the bottom of your post is not a G5 model iMac, and

there were no powerPC G5 processor-equipped Macs after model year late

2005 to early 2006; so if the computer in the discussion is a G5 it is not 24-in

and would be an Intel-based model. These use different architecture than G5

powerPC though in some ways are similar at least in general design.

This contains all ifixit repair guide for Power G5 iMac (no 24-in model exists)

https://www.ifixit.com/Device/iMac_G5

If you have both an Intel-based iMac and an older PowerPC G5 iMac, specify.

Don't know if a reset of the SMC or NVRAM may affect the symptoms or change

the visual problem in the display; an underlying hardware problem is a likely cause.

• Resetting the System Management Controller (SMC) on your Mac - Apple Support

• How to Reset NVRAM on your Mac - Apple Support

If this is your model iMac (24-inch Early 2008) the build model identifier follows:

Introduced    April 2008

Discontinued    March 2009

Model Identifier   iMac8,1

Model Number    A1225

EMC    2211

Order Number    MB325LL/A

Initial Price    $1,799 (2.8 GHz) $2,199 (3.06 GHz)

You may be able to find part numbers and search for repaired or new replacement

old stock graphic cards. A friend had one of these 24-inch 2008 iMac models and

it failed due to graphic processor; where she lived the BestBuy did not want to fix

it although it was bought from them with their extended warranty; not applecare.

So there are some out there in near perfect condition, but not repaired, still.

Not sure if the costs involved would be worth the value compared to a different one of

later build that may have no similar design build history. {Confirm the iMac's identity.}

Sorry that I don't have any new ideas in regard to this problem.