Cubase LE 5 Play back Issue

I have a zoom r24 that I use as an interface with Cubase LE 5. The first few weeks I was using it it worked great. Now it records but only plays 2 tracks at a time. It randomly plays back different tracks. The monitor button is off. I have the correct device selected under audio. I reinstalled the zoom drivers. The output is set to stereo for all tracks. Should I reinstall Cubase? Is there another setting I should look at? I have Win XP Pentium 4 cpu 2.80 GHZ, 1.0 GB ram. What would make it stop working properly?

Kind of difficult to work out whats going on from your description, could you try to explain whats happening so that someone with a blindfold and earplugs could understand.