Are you using 3Com nic card?
I have simillar problem, X lock up playing tuxracer and sometimes screen saver. Someone in IRC told me they tracked it down to the 3Com nic, with 2.4.18 kernels and nvidia driver.
This makes sense because with the 2.4.9.X kernel in RHL 7.2 I had no issues like this.
His solution was use an Intel nic. Not a good option for me because the two nics are built into my motherboard :-(.