How much RAM have you got in your system?
I’m not sure exactly what Cubase means by ‘disk cache’ or how Cubase loads audiofiles and such when you open a project, but I’d assume Cubase tries to load as much as it can from your HDD into RAM and once your RAM is ‘full’ (as in: Cubase is unable to allocate anymore RAM for audio files), it’ll have to page out existing data from RAM and load more data from (the much slower) HDD into RAM, which can’t keep up with the project playing back, thus causing the disk spikes.
So another take on this could be insufficient RAM. Are you running 64-bit Cubase? Might be worth a shot if you aren’t already, as Cubase 32-bit on a 64-bit OS still can only allocate max 4GB memory.