How can I tell if I'm being "throttled"

[#1]

I've been with Free-online for many years now and it's normally been OK (ish):  now I'm gettting really annoying cut-outs in my streaming of radio whilst recording.  OK, I know that I'm a long way from the exchange, I've tolerated 375kbs for ages now but this is annoying  I can't tell if stuff that I've been recording is thee until I listen to it 😞
Speedtester has never really helped before except to confirm what I know.
So - before i start down the long road of dealing with BT etc - how can I tell that it's not my ISP that's doing this to me?
BTW - I understand that Free-online = Pliusnet BUT Plusnet is owned by BT - so one would expect some sort of special relationship, but one never sees that?
cheers, TerryB

[#2]

Welcome to the forum, from another Free-online person

Plus Net is owned by BT retail, PlusNet gets their service from BT Wholesale, who have to treat all customers equal, BT Speedtouch operate the lines and the exchanges.
what is your line profile? https://portal.plus.net/my.html?action=data_transfer_speed  - dont print your phone number
are you mixing up your  kb and kB  ?
your package throttling is shown here http://kitz.co.uk/isp/plusnet_shaping.htm, but you still dont now how your downloads arrive, up to more of an expert than me