@bob434 You don't think it could just be that phones will drop automaticaly to a lower spec if they detect a weak connection.
It may take a few packets of data before a phone decides to try a different standard - at least thats what I've found.
Just like wifi, you can be getting a great signal (and data through put) in one spot and then just by moving a few feet get the opposite.
An example of this is where my sister lives, the whole house used to be a dead spot for the three network - one or two doors either side, no problem - and it would take my phone a while to work that out.
I used to see a decent signal, think they had fixed whatever problem it might have been and be dissapointed a few minutes later when the phone gave up.
No idea what its like there now, she has always used O2 and never had a problem, and I've not used three since soon after I moved to giffgaff.
Trying a few things out recently - just to get a measure of how stable data is where I am, I also found that several times, soon after getting home, the phone didnt have a very good connection - poor test results, slow web access etc. - re-booting the phone, or switching flight mode on then off fixed it and tests showed my 'usually ok, but nothing special' results.
From that, its a fair bet that while moving around, the phone could be holding on to the signal from a tower that might not always be the closest/best - how long for, I don't know, but I guess it waits a while to be sure that the new & better signal is going to stay in range before switching over.
Resetting or turning flight mode on/off makes the phone forget whatever connection it had and connect to the best signal it finds.
(at least, thats what the results I've been getting have me think. Incase it makes a difference, I don't use unlimited data as I sometimes need to tether a laptop.)
So, if you get the problem of speed dropping, it may be worth resetting the phone and/or trying different rooms if you're indoors.
Hope it helps.