It may to do with the way they configure their network.
Although the MSANs that TalkTalk use are actually adsl2+ compatible, as are the routers they ship out.. TalkTalk dont offer a true adsl2+ service and cap the speeds to 8Mb.
adsl2+ is a technology that allows higher frequencies to be used.. but the provider can still configure the DSLAMs/MSANs to restrict certain frequencies OR speeds.
However it may also be that just plain and simple that at the time of sync there are insufficient bits in that particular tone for it to be usable. Each carrier bin needs the equivalent minimum of 6dB SNR to be usable.
>> Sometimes they stay being used and other times it drops them.
That indicates bit swapping process, and that theres been a decline in your SNR ('real SNR' and not your overall SNR Margin) for those particular frequencies. As the SNR for those channels drop below 6dB, your router will attempt to bit swap those bits to use in other channels if it can. If it cant it will loose sync.
I could be wrong, its hard to say without seeing what your bitloading is like, but in view of the above I'd say its more like you dont have sufficient SNR for the particular channels to be used.