Bad news...have the latest echo drivers. Not been updated since 2/2010. However, it did get me to read the "read me" on the driver set...it says under known issues "if you experiences glitches or distortion while recording, disable unused inputs/outputs in your DAW software". So, I have disabled the ADAT IO, which is only occasionally used. Can't rerun my test though, as it uses the extended libraries, which apparently just ran out. doh! Since my real world projects are not glitching since the update (only the stress test project I made)...I've got no way to see if that helped.
I also found a setting in Cubase to boost the audio priority. I've set that to boost.
Also on the graphics card...didn't mention--the drivers for the Quadro cards are meant for more window and openGL acceleration (intended for CAD use) versus the GeForce line. I know the CAD engineers at work require Quadro and FireGL cards even though the "consumer" lines are technically "faster". avid only certifies Quadros for use in HD rigs, too.
What CPU are you running with the 9800gt? Do you see the CPU jump significantly with the GUI open? I'm running a fanless 9500gt/1gb with a core2quad 2.5ghz. Martin's testing with a i7/Quadro. I do also believe its using the CPU rather than hardware acceleration. However, the problem gets far WORSE with aero (thus graphic acceleration) disabled.
Im going to continue to use it with the newly disabled IO. Seems like a weird thing that disabling IO that aren't being used would change performance, but it does say that in the driver read me, so...We'll see...once I actually get the money together to buy the extensions, I can rerun the test project.