I came across the same pages you came across saying that the problem is either the file or the physical ram.
This said tho, you should also try running a scan disk. (prefereably on a diff machine if possible). If when you run a scandisk, you get a LOT of hd errors reported you probably have a RAM problem... especially if you do a 2nd scandisk and get the errors in a different spot (assuming you didn't fix errors).
I think that the best thing for you to do is run memtest86 (
http://www.memtest86.com/) to check if the ram is ok. If that checks out then try replacing the himem.sys file.
note: I have prevously seen where a hd has a bad block on part of a system file and it will randomly work/not work depending on how windows loaded up that day.