You might have to run defrag on the D Drive after each day of project saves, might help.
If you are using VSTis with large libraries any slowdown due to defrag will affect the streaming from disk and you get the overload.
You could try expanding your disk caches. I’ll have to check how that’s done because I’ve forgotten but I also feel that it may have negligible benefits. I’ll also have to check whether it’s worth doing or even possible on W7. I feel it’d be worth a couple of minutes reasearch on it though.
I’d say get more ram but it looks enough. It might be an idea using the W7 Task Manager to check your drive usage to see if there are any unusual spikes. If you leave the task manager open it will show up at the W7 taskbar bottom right where you can access it while using Cubase.