SPEED PROBLEMS WITH PIXELVIEW CAPTURE VIDEO CARD

Hi, I'm work with D4 and a pixelview TVpro capture card. I see this problem:

When I move a window under the camera window (capture window) the image go
slow, and the system too !!!
I will try to send messages like capture to clipboard and capture to disk
but the only solution is click with the mouse on the cap. window and then
press the F2 key to capture to clipboadr. In these case the image speed
return to normal .
The driver is for Bt878.
The same problem hapens when I change from Preview to Overlay video mode.

Any body knows how to capture in Overlay mode from the memory card buffer ??
Is it possible ?? With the goal of increase the capture speed.

THANKS IN ADVANCE !!!!!!