repeated HD issues

Last week my 120GB Seagate drive died (no warning, rainbow cursor, question mark folder on restart).
I attempted to recover it by starting up from the Leopard disk and using Disk Utility, but the drive didn't even appear.

Then I tried Disk Warrior, same deal - didn't see the drive.

Having given that drive up for dead I purchased a Western Digital 320GB. I formatted and partitioned it, then installed Leopard and other main software (Logic Pro etc). Worked fine for half a day until I got the rainbow cursor and had to force a shutdown and restart.

After that it wouldn't boot up completely, getting stuck on the Apple and spinning icon or the blue screen. So I repaired permissions and verified the disk via DVD ...all apparently OK.

I then tried an Archive and Install - after which the computer booted up fine and worked for an hour or so before I got the dreaded rainbow cursor again for no obvious reason. Restarted again and ran Disk Utility on the drive. This time when I tried to Verify Disk the verification failed.

So I did an Erase and Install, re-partitioned the drive and loaded a bit of software. Now things are generally running OK but I occasionally get the rainbow cursor out of nowhere, either slowing things way down for a brief period and recovering - or staying on so I can't do anything and have to force a power down and restart.

When I run Verify Disk now it comes up as either all OK, or fails to verify and tells me there's a problem with the drive. Very confusing...

I'm thinking I might try another new drive - but am concerned that this might be a hardware problem, in which case I might need to send it for service.

Is this a HD or computer issue? Any ideas?

Cheers
Duane