The problem you describe is a classic symptom of insufficient signal strength.
Even though the strength of signals broadcast by the transmitter remains constant, the strength of signals received by your aerial can still vary greatly. Weather, atmospheric conditions and solar flares can all adversely affect the signal you receive. In particular, high pressure (especially during the summer months) affects the Earth's ionosphere, with subsequent problems for the reception of rf (radio frequency) signals.
Your aerial (and the quality of its cabling, which is just as important) is good enough to 'bring in' the signal during ideal conditions, but when things change for the worse (as with the current high pressure) it ceases to be so.
Ideally a TV aerial should have elents which are exactly the right length to 'resonate' with the incoming frequency. Since every channel is on a different frequency, that would mean having a different aerial for every channel that you watch. That's clearly impractical, so we each use a single aerial, which will be better 'matched' to the frequencies of some channels than to those of others. Your aerial is clearly rather poor at receiving the ITV & C4 group of channels.
Since you get a good enough signal most of the time, a simple (cheap) signal booster will probably fix your current problem. If not, you'll have to get an aerial fitter to supply a new aerial and cabling.
It's also worth remembering that (except in those areas where the old analogue transmitters have already been switched off) all Freeview transmitters are currently operating on reduced power. (That's necessary in order to avoid interference to analogue transmissions). So reception problems are currently more likely than they will be after all analogue transmitters have been switched off.
Chris