Hello,
a month ago somebody asked me about the same problem. He was
running Mandrake 9.0 on his laptop computer. He solved this problem
by installing Mandrake 9.1 but he wanted to know what was wrong
with his old installation. At the time I was running the same version
of KDE on a DELL Inspiron laptop and never saw this problem. I recommended him to stop all the network services like autofs, netfs, nfs, ftpd, ypbind, cups, etc. Only this way the system was able to
run with no slowdown but I never found what service in particular was
causing the problem.