Hmmm..
If possible, try attaching a computer monitor to the splitter (with tv attached) to see the image it's passing along to the converter.
If its clear, then I would suspect the hdmi2av converter.
Might be able to test the hdmi2av converter (and grabber) with something else that output's video with hdmi. A dvd player or a laptop with hdmi out, even a desktop computer.
Could test the usb grabber with something that output to rca style connectors. A dvd player or camcorder maybe.