Modem speed is half

Hello, I am currently on dial up.. and every bit counts. I normally connect
at 45,000 BPS, and now its 28.8 BPS.

It just suddenly changed.. (it did this before vista too)

Heres the options I tested:

a.) The phone line. phone company ran diagnostics and it ran fine.
b.) A different phone line. Same thing happened. half speed.
c.) different cord to connect. Same thing happened.
d.) New modem of the same brand. Same thing
e.) New modem of a diff. brand. Same thing
f.) New Drivers? there from 2003, But they work with vista, and the most up
to date ones are 2003.. If anyone could look into this.. my modem is a V.92
PCI.
g.)Different computer- I tried using a different computer, and guess what?
It ran at full speed...
h.) Reformat. I reformatted my entire computer incase of viruses. I tried on
XP, and then Vista.. still half speed. Back with xp it said something was
blocking my connection... But I never figured it out. and as I said, Vista
says theres no problems
I.) ISP- the other computer has the same ISP
J.) The number I dial into is the same on both computers, yet mine runs at
half speed.
K.) Diff. Ports- I plugged the modem into every single PCI port.. same outcome

These are some things I think it could be.. Motherboard glitch? Power
supply? Any ideas? sorry about all the reading
Something wrong with my motherboard?