Xena LS and video effects in Premiere Pro CS4

I'm using a Xena LS (SD) with Premiere Pro CS4 and I can't apply video effects to clips.

I'll explain using a basic RGB color correction:
- After I apply the effect to the clip, I go to effects control.
- I expand a color corrector, i.e. red.
- When I move the cursor to a high value of red, the clip turns bright red as long as I keep my finger on the Left Mouse Button.
- As soon as I release the LMB, the color goes back to the original color. The value remains as changed, but the color correction is not retained.
- If I click again on the color corrector cursor, the clip turns bright red, but as soon as I release the mouse button, it goes back to the original color.

I posted a thread on the Adobe forum (Premiere Pro / "CS4 video effects" thread).
I had great help, and the consensus is that the problem most probably comes from the Xena card.
Of course, it's not 100% sure, so I'd like to know if anyone else is using the Xena LS with Premiere Pro CS4.

The workstation is a Dell Precision T7500 (Quadcore 2.5 MHz)).
The card is on a PCI-X 64-bit slot.
Graphic card: NVidia Quadro FX 3800
6 GB RAM
External HDD: Micronet SCUSI 320. DMA test shows 134 MB/s read/write.

My footage is SD (Betacam SX) playing at 27 MB/s.

Any input is welcome.
Thanks!