Help Data often fails unless on/off/on again

[Intelliphone]

My data connection has become unreliable. The H symbol appears but nothing loads; I have to turn data off, then on again; it then briefly shows the G symbol, then H, then it (sometimes) works. Other times it still doesn't work, but when it does, this is always the process.

I'm on Three, but a friend with an S4 on T-Mobile has the exact same issue.

Is it searching for different masts?

I've not changed any settings and rarely download any apps.

Any help appreciated - fast losing patience with the S3!