Two factors. The tuner in USB TV dongles is not all that great and USB 1.1 bandwidth is very low. The USB TV dongles often degrade quickly as people DROP them and the tuner takes a jolt.
Fix? Get an USB 2.0 device and don't drop it.
Can somebody tell me why, in the name of God, I can see my recording in the TV in great quality but when I watch it in my computer through the USB it looks as if someone was puking on it?