You did adjust the com ports speeds to the highest possible? its 921,600 bits per second, 8 data bits, No Parity, 1 stop bit, flow control Xon/Xoff. adjust the FIFO buffers to Receive High (14), Transmit Buffer High(16). Adjust your modems to highest possible speed - 115,000.
Also make sure you are not running the phone line across any AC power cords. Make sure the the modem, i am guessing it is an internal model, is properly seated, and that the retaining screw is in place. this will keep the modem from moving in the PCI slot, and making poor connections to the slot.
Check the FREE phone wire they ship with the modem, and replace it, its usually a very cheap wire and low gauge wire and sucessible to noise.
replace the phone line from the wall to the phone switch box, before you replace the line to the pole.
make sure the phone jack is seated properly and tight into the modem and the wall jack.
make sure you dont have any old modem drivers on the computer. only use the newest and updated modem drivers.