My Apple G5 using operating system 10.6.8 keeps locking up. I get no warning or error message. I do get some intermittent screen distortion. Any ideas as to what could be causing this and how i get rid of this problem? Complete system re - install? N

A G5 is a Power PC model and cannot run 10.6.8 Snow Leopard.

That is a Mac Pro, not a G5. 🙂 It's from 2006, most likely. When it crashes what do you see on the monitor? Before running out for a new GPU I would backup then erase the startup volume and reinstall Snow Leopard. See if the situation improves. If it doesn't, then get a new GPU. I recommend looking for a Mac version of the ATI HD5770 card. Anything after that isn't worth the money.

You should get a Mac specific version. And it must be one your Mac model supports. I think your best choice for that Mac is an ATI 4870 HD. The main difference between it and the original Mac 4870 is the first ones had two DVI ports. The one I linked to has one DVI and one DisplayPort output. DisplayPort handles higher resolutions with higher refresh rates. Another genuine Apple 4870 HD for $135.

There are a lot of 4870 cards meant for PCs that have been flashed to work in a Mac. While they may be okay, I personally wouldn't touch one.

According to everymac.com, your Mac came with Tiger, 10.4.x and can't go any higher than Lion, 10.7.x.

It would have come with gray, machine specific disks. Since there's no reason to install 10.4.x first, and then Snow Leopard over that, just purchase a retail copy of Snow Leopard for $20. What you won't get with the retail version of SL is the iLife suite (Garage Band, iDVD, etc.) that were on the original bundled disks, but you can purchase iLife on Amazon pretty cheap, and get newer versions besides.

When you get Snow Leopard, you can boot to the DVD, erase the drive and install everything clean. Hopefully, you issue is only software, and not hardware related.