I have the same problem. Sadly, it's traffic related and not particularly Tiscali's fault. I was with aol for over a decade and switched to virgin. For the first 24 hours I was wizzing along at around 5MPS..but ever since then after 4 pm, the speeds run at around 300-600k...until after midnight.
I suggest that the isp's do this intentionally to allow all users to have at least 'some' access continually...with the advent of streaming, on-line gaming, video and music downloads, the traffic is bulging.. But of course, you can correct this by purchasing a higher premium package from Tiscali...Virgin..or anyone else...it's down to 'ya get what ya pay for.'
have a look at
http://www.broadbandwatchdog.co.uk for intersting information and speed tests..
Best of luck
Fr Bill