I was having problems since my firmware updated to 1.02.28. I live in North London so I am served by the Crystal Palace transmitter. i traced the problem down to Channel4 +1 being set to another weaker mux. To get around the problem I manually retuned (Menu / Settings / Installation / Manual Search) channels 22, 23, 25, 26 and 28. That was probably overkill, I think 26 was the only one that need tuning but the problem is sorted.