GRUB gives me Errors 16 and 18, but boots successfully after a few tries.

[Adam Funk]

Booting often produces "GRUB loading, please wait..." then "Error 16"
or "Error 18"; I hit Ctl-Alt-Del and it might do one of those again,
or boot automatically, or drop to the GRUB menu.  Selecting a kernel
sometimes throws one of those errors, but if I try again, it often
works.

The computer is running Ubuntu Feisty and is just over a year old, so
I would expect the BIOS ought to be able to handle a "large" hard
drive (this one is SATA, if that makes a difference).

I'd appreciate some suggestions for identifying or fixing the problem.

Thanks,
Adam