It sounds like inadequate signal strength to me. With some Freeview TVs (and related devices, such as set-top boxes or PVRs) you'll see the picture 'break up' when the signal strength falls below a critical threshold. But with others (particularly the latest models) the screen simply goes blank (and/or the sound drops out) when the signal isn't good enough.
At this point I can hear you saying: "But my previous TV never had any problems like this". It should be noted that different brands of TVs (and digiboxes, etc) have different 'sensitivities' (i.e. their abilities to cope with poor signals). It's likely that your new TV has a lower sensitivity than your old one did.
Next I can hear you saying: "But the problem is only there occasionally. How can it be related to poor signal strength?" The signal received by your aerial doesn't remain at a constant level. It's affected by weather conditions, atmospheric pressure, flares on the surface of the sun and many other factors. Additionally local interference (such as when your next door neighbour switches a light on or off) can be sufficient to cause the loss of your picture for a few seconds when you only had a 'borderline' signal to start with.
Since your aerial is already providing a signal which is good enough MOST of the time, you probably don't need a new aerial to upgrade to a system which provides a decent signal ALL of the time. I'm prepared to bet you twenty quid that simply adding a cheap signal booster (less than a tenner from Asda, for example) will fix the problem.
Chris