You should tune to that channel and see what the signal strength and SNR is for it under Messages & Settings/ Account & System Information / DVR Diagnostics on that channel.
Signal strength under 60 can get blocky/blotchy and even though 100 may be the max, it can get really strong and cause problems.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!