I`m using Wanadoo/Orange 2Mbps with OE and have had no problems although I have seen a number of posts on the subject. Must also admit I send relatively few e-mails and receive very few as well (thankfully virtually no Spam). Main contact is with family and mail seems quick enough both ways.
Have you tried using OE instead of Thunderbird just in case there is something being caused by the software? I know it`s a longshot but maybe worth a try.