Broadband Fibre speed drop problem

[#1]

Hi folks.
Signed up to Plusnet Fibre unlimited upto 40 mbs. Works great, However over a period of a month the speed drops off from 40 to 45 mbps to 10 mbps. If I reboot my router it returns to 40 mbps. Any ideas why this happens?
My theory is Plusnet save there broadband traffic but sneakily dropping the speed of the service I pay for.
It's getting annoying so looking to change provider as they are not providing the service i'd expect from the one I signed up for.

[#2]

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My theory is Plusnet save there broadband traffic but sneakily dropping the speed of the service I pay for.

I can assure you we don't do that, there'd be no benefit from doing so as it'd annoy customers.

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It's getting annoying so looking to change provider as they are not providing the service i'd expect from the one I signed up for.

I've just taken a look over your account and can't see that you've ever reported this to us to look into? Unless we're made aware you're having issues there's very little we can do to help.
If the same issue happens again please let us know so we can look into it.