Mac Mini slow when accessing hd - NVidia MCP79 SATA Driver Problem?

Question

I bought a Mac Mini (Early 2009) a while ago. Unfortunately i have a problem concerning speed and responseability which i couldnt solve even with a lot of googleing.
I noticed that problem first time when i tried to play a video file from hd. The video does not play as smooth as it should. It stutters from time to time. It is like a short interruption. The picture freezes for half a second and replays the lost frames very fast after that. These freezes do not occure periodically. It seems to appear randomly. The wierd thing is that the cpu usage is very low and there is enough free ram and disk space.
So i think its not a ressource problem. I noticed that these freezes do not only affect video playback. The whole system is kind of unresponsive. And thats the the case if there is some sort of hd or dvd access. When i try to copy a huge file these freezes are increasing rapidly. The more hd access the more the system is unresponsive and cannot be used anymore.
All these effects reminded me of a faulty dma/udma setting on windows machines. So i tried to check the transfer mode for my macs hd controler. But i didnt find any place where to check that. And i dont really think it is running in pio mode because that would result in a high cpu usage.
I played around a little bit to understand whats going on. So this is what i found out:
When i play a video from the hd its stuttering. It increases if i copy a file from or to the hd. Copying a file from the dvd-rom increases the effect as well. It is not influenced by a copy from AND to an external usb hd. And the whole effects do not appear under Windows Vista no matter how much i acces the hd.
So i come to the conclusion that there must be something wrong with the SATA controler driver which is a NVidia MCP79. Unfortunately i have no idea what i can do to solve this problem. And i didnt find any advice or something in the internet.
So i would appreciate any suggestions.

Answer 1

Ok, I think I was jumping to conclusion. Disabling journaling didnt remove the problem. It just seemed so because my problem is much less distinct with Leopard. Bad luck I guess.
kmac1036, to answer your questions. For sure I dont have warranty anymore. Maybee I should have posted that I upgraded my Mac to 3 GB RAM. Furthermore I replaced the original Hitachi HD with a Western Digital 500 GB model.
Somehow that may be connected to my problem. I've read that many people have problems with the WD HD and those beachballing issues with SN. (I do so too)
But my performance problem when accessing the HD did also exist with the Hitachi. In fact the Mini was extremly slow with the original 1 GB RAM. I think putting some additional memory into the machine helped because it reduced hd accesses.
Ok, I am not an expert. But if I had to guess I would say the following seems to happen. There may be an operation in the hd driver that takes longer than it should. Because of that it maintains cpu cycles and prevents the kernel to continue scheduling. That would explain why the freezes are not connected to a high cpu usage and why the systems seems to catch up things after the freeze very fast.