It sure doesn't sounds like you have 50% - 75% signal strength. I think someone was saying the other day that this indicator is far from accurate which is quite believable. Besides if the problems you are seeing are spurious then the 50% - 75% numbers might be a maximum reading rather than a minimum one.
With pixelation, skipping, lost channels and "No Signal" messages you are certainly giving strong evidence that you have either a wiring problem or a dish misalignment that may or may not be causing problems when recording.
Although I don;t yet have Sky+ (next tuesday

) I can well imagine that if you are recording a programme during which time the signal is too low or drops out completely, even for a split second, the Sky+ box may react in some unexpected way. From its point of view why should it continue to record while the input signal has dropped out or reached an unacceptable level.
You might have two problems rather than just one, but I would suggest having a close look at all your connections into the Sky+ box before going any further. Check that the coax cabling has a good connection to the box and that the end connector itself is well wired to the cable.
Good luck.