I had a similar problem, also with an IBM laptop (a Thinkpad A21p) with a 1600x1200 LCD panel and an ATI Rage 128 Mobility M3 LF (according to XFree86) chipset. I fixed the problem by installing an XFree86 4.3.0 server using the xfree86-server 4.3.0-0pre1v3 package from experimental.