See if you can find a copy of the program called "Hauppauge Tweak Tool "
I can't seem to find an active link.
There should be some options to turn off the spatial filters and adjust the motion blur of the card's drivers.
My PVR-150 used to have the same problem, and would also give me a very dark picture as well. Just play around with the settings to see if you can improve your picture.
Don't worry about screwing things up. Worse case, you return everything to the default setting or just reinstall the drivers.