I don't know why, but it's hit and miss. Personally, I have no problems at all with HT and APCI on both at the same time. My system is running really, really well right now using Intel 875pbz board
I did quite a long post not too long ago with my system specs.
I have a problem with Cubase crashing. I wonder if I can get this PDF file in english. Or should I get a USB midi interface instead. Come to think of it, my interface has USB as an option. It's an Emagic Unitor style. I'll have to try this.