I was just coming on here to ask the same question. I've just bought an 01 4.4i and it has a judder like you describe. I didn't notice it on the test drive though.

I would describe it as the car being indecisive as to what gear it wants. It only does it for a few seconds if I'm going light on the gas and somewhere between 30-45mph. If I put my foot down it doesn't happen.

A mate of mine says "that sounds expensive" when it happens, lol. I've only had the car for a week so hopefully as Fourpointfour says, it's normal.