You've described the classic symptoms of insufficient signal strength. Different makes and models of TV sets have different 'sensitivities' (= the ability to 'lock onto' low strength signals) and different 'selectivities' (= the ability to reject interfering signals). So your new(-ish) telly might be more susceptible to problems with weak signals than your old one was.
Further, the signal strength at your aerial doesn't remain constant. It's affected by the 11-year sunspot cycle, individual flares on the Sun's surface, atmospheric pressure, precipitation and loads more things. So, if your telly only has a 'borderline' ability to handle weak signals, there will be good days and bad days.
Start by checking that your aerial appears to be in the correct position. (There was one thread on AB, about a decade ago, where I suggested that the member should check on his aerial and he found it lying in his front garden!). Then check that it's probably plugged into the back of the set and that any other connections in the lead are properly secured. Then go out and buy one of these
https://www.argos.co.uk/product/9383027
together with one of these
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720
Install them, retune the set and everything will probably be OK again.