How to test sustained I/O throughput?

I've been having a problem with my imac for quite some time.

It's an iMac7,1 24", bought late 2007. It has 4GB RAM (2GB added by me). Sometime last year it started randomly "pausing" (best description I can come up with) giving the spinning beach ball when trying to do stuff. I figured since the stock HDD it came with was 99% full, that it was likely due to the full HDD and so decided to upgrade. Had an authorised Apple service centre install a 1TB drive for me. Since then, the trouble has continued and has slowly gotten worse.

Recently took it to get repaired. Initially they could not reproduce, but after I showed them videos of it misbehaving and then demonstrated for them in the shop, they eventually said the logic board was at fault and replaced it, and told me that seemed to fix the issue.

However, the issue still persists (just as bad), and I am about to take it back again to get them to look at it anew.

The symptoms best I can tell is that occasionally when it goes to fetch a file from the HDD the I/O throughput drops to zero or very near zero, resulting in the extended delays (while the UI itself stays responsive).

How I want to test this is to run some kind of test utility that repeatedly reads/writes smaller and larger files over a sustained period, graphing the throughput, to see how often it freezes up, so I can reliably demonstrate the problem and have a reliable test to see if it is fixed.

Anyone know of any software like this?