ajobbins: This was a major issue a few years ago on the Vodafone network that had these exact symptoms. JohnR also vehemently denied at that time there was a problem and it look many months for him to accept otherwise ;)

It was fixed back then, but could be back perhaps.

I haven't seen it on 3G at all lately, but haven't been close enough to the city to see if 4G is still a problem.