I found a similar bug in Debian:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=356863
Machine is Fujitsu Scaleo 600 there.
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Regression: grub - Error 18 sometimes since edgy
https://launchpad.net/bugs/79578
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ubuntu-bugs@lists.ubuntu.com
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Since I installed grub from Dapper no more problems appeared.
BTW
I never had any problems like this with hoary, breezy or dapper on this machine.
Some additional info:
Mainboard/BIOS:
ASUSTeK Computer INC.
MED 2001
REV 1.xx
Award Modular BIOS v6.0
ASUS MED 2001 ACPI BIOS Revision 1009
06/12/200