Why does my mac stop downloading files, copying to clipboard, saving files to desktop

[nomenculture]

Why does my mac stop downloading files, copying to clipboard, saving files to desktop, etc?

iMac g5 1.9 ghz 17" running 10.5.8. Had a hard drive crash and bought a new one. Loaded new OS leapord. Everything works fine at first, but after the computer has been on for a while, it will stop downlading files, disk verify doesn't work. I can't copy and paste stuff. It seems as if the hard drive is full, but I have only used 32GB of 300GB.

When I restart, it works again, then slowly not. Is it an overheating issue? How do I know if my fan is running? Is it a bad hard drive I put in it? Bought it new with three year waranty.
What's wrong? Please help.

 

[gjw4u]

Why not use iStat and the 'Activity Monitor' to see if the G5 if overheating or the CPU is maxed out.

 

[nomenculture]

Why not use iStat and the 'Activity Monitor' to see if the G5 if overheating or the CPU is maxed out.

What do you mean by maxed out?

 

[gjw4u]

nomenculture, with maxed out, I mean that some process might be hogging 100% of the CPU. Using the 'Activity Monitor' you might find a culprit.