This is the joys of routers, same line and a few routers under review at present
1. My usual one is rock solid
2. Another great speeds but less stable, but has lots of tweaking options
3. Another that seems to not as stable as first one, but better than second.
So even with the same line and same ISP you can get varying performance from hardware. So while changing providers can be a solution, it is worth looking at hardware options, or improving line performance, particularly where someone may be locked into a contract.
The TalkTalk Huawei modems do seem to hang onto a line down to very low margins, with throughput suffering, i.e. pro-active restarts can improve things as can switching between ADSL modes manually.