Personal Hotspot Problem

Hi Everyone

I wonder any of you can help with this one?

Whenever the Personal Hotspot function came out(a couple of updates ago)

it has always done strange things on my iphone 4.

When I hit the settings button, it sometimes does not appear on the menu list, sometimes it does(I have never used it).

Sometimes it is 'grayed out'.

Now with iOS 5 it goes between being 'grayed out' and normal but when I try to switch it on, it will not come on but I am able to change the password.

It seems as if it is corrupt and I am wondering if anyone knows how to sort it?

Thanks in advance

Charles