If you want to put the hammer to it and really heat them up try memtest. Let me see if I can find the post about it I did here.

Here it is revised.

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memtest Not free but $1.39. I've had it find find bad RAM when Apple's hardware test couldn't. Run it in Single User mode first, that way it only uses about 50M to boot up and will test the majority of the RAM. Processor caches are disabled in SU mode, so If you double click memtest while running from a regular boot it will also test the processor caches, but less RAM, since it takes more RAM to boot regular.

A way to get it free is to download Rember, then go to Show Package Contents for Rember, Contents/Resources/ and copy the memtest file out of there onto the HD. Here's the user's guide. You don't get it with Rember, only with the memtest d/l version, guess it's worth the $1.39 for the guy that wrote it. Memtest is open source.

Copy memtest to the top level of your HD, and to run it boot to Single User mode, then type /memtest*all*6*-L. * = space. /memtest invokes the app, all tells it to test all available RAM, 6 tells it 6 passes, -L puts a log on the top level of the HD.

It takes about an hour or so on my dual 867 to run one pass of 2G of RAM. You want to run a few passes to really heat up the RAM to i ncrease chances of showing marginal RAM. I've had it not find bad modules on the first pass or two. Any error is not good. If the machine locks up it's not good. If it does remove one module and retest. It doesn't have the ability to tell which module is bad.