Kernel update driving me mad

[Klaus-Peter Schrage]

... (not really, because I have a good old kernel installed to fall back on).
In brief, what I did on a Redhat 7.1 system which is very up to date as 
patches recommended by RH are concerned:
- download the latest securrity update kernel from redhat.com
- checked md5sum: ok
- rpm -ivh kernel-2.4.9-31.i686.rpm, like I did many times before with other 
kernels (2.4.9-21 was the most recent)
- change grub's config file menu.lst to point to the new kernel
- reboot.

Now booting into kernel 2.4.9-31 hangs after the message
        "Unkompressing Linux ... Ok, booting the kernel."
is displayed, and my machine can only be resuscitated by pressing the reset 
button.
I have checked again and again, it's not a question of typos in menu.lst, the 
new kernel IS recognized by grub.
There are two more baffling observations:
1. That very kernel boots from a 1.44 boot disk
2. I downloaded the corresponding headers and sources too and did a 
recompile, applying a known good .config file that a have used before with 
other kernels from the 2.4.9 series: No problems in compiling and installing, 
but the resulting kernel 2.4.31custom hangs again.
Any ideas, perhaps I'm too blind to see?
Klaus