Strange DSL/dial-up interaction??

hi guys

windows-xp-home, DSL internet on separate line from phone, speedstream 4100 modem from pacbell/att

2 weird things:

first the dsl will sometimes slow down a lot (from 1300 kbs to 300 kbs).

then, when i start a dial-up internet connection, the dsl speed suddenly fixes itself, jumping back up to 1300!

how could they relate to each other like that?

of course i don't know what causes the dsl slow-down either! i got a new modem a little while ago so it's not that.

thanks in advance.