It might just be an issue with Tascam and/or Cubase.
I've got both a 1800 and an 800 and Cubase 6. Open virtual instruments seems to spike it. Really weird. Doesn't happen in Ableton for me. Or in the standalone versions of those programs.
"Back in the day" when I used to use Le5 I didn't have the ASIO problem. It's only began as recently as December for me. And Cubase also causes my computer to BSOD at the use of the freeze function. Fresh OS installs were done, latency settings of all kinds, and everything. I just don't know. It sucks because I love Cubase.
Same issues have happened to me on 3 completely different (but optimized for audio) systems. I'm not sure if it's a Tascam/Cubase compatibility issue (since I've only got Tascam interfaces) or if it's just a Cubase issue. Though I know that Jimmy here uses an 1800 and Cubase and I don't believe that he has any problems.
It's all very strange. I feel your pain though.