It sounds likely, which would make it a motherboard problem as they suggested - though perhaps not one that requires replacement or repair. If you aren't using all the slots and it works fine this way, then I'd say leave well enough alone and be thankful you've found a solution. Is EVERYTHING working fine now that you've done this? No problems of any sort? Then I'd say this issue is resolved.
Good work on tracking down the problem and solving it!
If you're still interesting in testing the RAM chips, the following was buried in my original post and is a good way to test them:
You can also test your RAM at http://www.memtest.org/. If it fails, test each chip individually to find those that are bad and replace them. If it passes, RAM is probably OK. If you consider the issue resolved, don't forget to close out the thread by clicking the far right triangular icon at the top of your post and typing "solved" in the box and submitting it. The moderators will notice and put a green check next to the thread title. That way others seeking answers by searching the forums will know that this thread contains a solution to the issue identified in the title - and people seeking to help will not waste time checking out this thread since it has been resolved. If it isn't resolved, please clarify how we may help. And welcome to Vista Forums! Now that you know where we are, bookmark the site and post anytime you have further questions or problems or if you just want to browse and learn (especially the Tutorial section offers some excellent advice on a wide variety of topics) or even if you want to try to help answer a question if you happen to see one where you know the answer and it hasn't already been posted. While it most likely won't be me the next time (though it might), there are many very skilled, experienced, knowledgeable, and friendly people here who, like me, volunteer their time to help others and one or maybe even several will respond to your post to assist you. While there are other forums on the web that do this sort of thing, you've now found what I think is the best (and I've posted in and/or checked out quite a few) so don't lose this link - you never know when it might come in handy again. Good luck and best wishes.