Just some background, I used to be on Sky's mid package but had no end of trouble with slow performance - mainly because I'm far away from the exchange (5km I think!!). I never got more than 512k connection. However one day after rebooting my router I got 2meg and it then degraded again. Sounds like the line was adapting to error rates etc.
Anyhoo after suffering from poor performance I switched to Metronet and I have had a rock solid 3mb download ever since.
So am I tempting fate by switching to say BT ? Will I be back in world of pain 512k??
Switching to BT specifically shouldn't affect your connection speed, but it could severely affect your download speed!
The original change from 512kbps when it suddenly gave 2mbps connection sounds like they moved you from a fixed speed service that was all there was available initially to ADSL Max "up to 8Mbps". The line adapting fairly quickly downwards after that is normal on long lines.
If you can post your line stats we may be able to help improve the condition of connection at your end.
See here for help finding them.
See
this comparison of what others think. Metronet belongs to Plusnet (possibly Madasafish which in turn belongs to Plusnet), and they all belong to BT Retail.
Do you plug the modem/router directly into the master socket, or use either an extension socket or plug-in extension, or even both? What modem/router have you got?