I had exactly the same problem and SKY would no nothing about it.
I moved to BE* and got rid of the problem!
SKY are, as has been stated, a mass market ISP and put stabilty over performance to minimise the number of support calls.
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity soon!