using DIGI multiport card (AccelePort Xp) with .NET2 SerialPort class in c#

i’m using .NET2 class called SerialPort (using System.IO.Ports)
when i open a serial port (let’s say com3) on the multiport card and then bytes are recieved on that port - CPU is getting into 100% and stays there!
even before i actually do the reading.
when i use the computer serial port (com1) - this doesn’t happen!
any suggetstions?