Perfectly normal to fluctuate between 5.3Mbps and 62kbps? Think not. It should be more stable than that.
SIN386 might prove to be quite interesting bed time reading for you.
It carries the rather exciting title of "BT IPstream Office, BT IPstream Home, BT IPstream Max & BT IPstream Max Premium Products - Service Description and Interface Specification".
In other words, it sets out what BT considers to be contractually normal as far as that product goes.
Amongst other things, it contains the "official" version of the table above, or at least as close to "official" as you will likely get.
Freely available from SINnet here....
SINet
P.S. For those not in the know, when BTWholesale provide the underlying delivery mechanism to most broadband ISPs .... therefore the above SIN document will apply to any ISPs using BTWholesale to deliver their broadband product.
one would think that contractually the supplier should at least provide what you're paying for
Indeed, and I've used broadband enough to know where you're coming from. But the devil is in the fine print .... regrettably as far as BT are concerned, if you're paying for "rate adaptive" then you're getting what you pay for.
Suggest you look at LLU Broadband providers if you have one/some in your local exchange...... they only rely on BT for the copper to the local exchange, and therefore have much more control over speeds and service quality when it comes to the service itself (asusming no issues with the copper !).