OK, I generated a modeline for 1400x1050@85Hz with 'gtf' and replaced the existing "1400x1050"-modeline in my XF86Config with the new one.
Unfortunately this didn't solve the problem.
Nevertheless, thanks for your help.
I don't know whether it is important: I'm running an LFS-System (www.linuxfromscratch.org
) with Kernel 2.4.20