Hmm, how is channel 5? Is that also jumpy and pixellated? Ditto for ITV2 & E4?
Other channels are OK? If so then there's nothing you've done wrong.
If it's just ITV/C4 & the above then it sounds to me like you have issues getting a strong enough signal to mux 2. What you may want to try is to get a signal booster from Argos like this: Aerial Booster
Without going too geeky though this may not work hence the suggestion to get it from Argos so you can take it back if it doesn't. If it doesn't then you are probably looking at an aerial upgrade which unless you can do it yourself will probably mean a visit from a local aerial guy. Looking at £120+ then by the time you've paid for a guy to go up the ladder and nail a big ass aerial to your roof.