As everyone else has said USB 2.0 is no use for editing despite it’s theoretical speed. HDV is VERY processor intensive the fact that your clips play back fine and then don’t when you apply a filter suggests that the CPU can’t handle the dynamic RT, providing you have set dynamic RT to be on. Fact is that an iMac is not the best system to be editing HDV on, a used dual G5 2.0 would have cost the same and edited much faster.
Having said that I regularly edit HDV on my 20″ iMac G5 2.0 at home, it’s very slow but once you know the limitations it does work, but forget any RT playback of filters.