I'd bet the cheaper tuners have almost no compensation for a degraded signal which is why they fark up so much and so suddenly. Whereas the more expensive tuners probably make a bit more effort to extract the signal using multiple signal filters plus they probably do a bit more work at the actual decoding. Or something. There's probably a world of things you can do for a few pence more that would improve the disintegrating image problem, that'll be the same few pence they choose to save on the cheapo machines!