How to test memory?

Try Rember. http://www.kelleycomputing.net/rember/

Use the Apple Hardware Test that is on the Install DVD:
http://support.apple.com/kb/ht1509

Bad memory will generally cause a kernel panic (and a reboot) - and not a short system freeze.

Does your cursor turn into a spinning beach ball during the freeze?

Software-based memory tests can prove that memory is bad, but those tests can't prove that memory is good.

The only definitive test for memory is to replace the memory, and if the problem goes away, then the memory was to blame.

Don't know how you ran Rember, however, you really want to set it to run multiple times, and let it run overnight.

As stated: Apple Hardware Test that is on the Install DVD is the best way

Rember or similar that run from the Finder cannot test ALL of the RAM because Mac OS X reserves a portion of RAM for housekeeping.

The 8Gb RAM in your mini may cause random crashes etc. if not 100% compatible. Apple does not offer a mini with 8Gb RAM so are you sure that amount of RAM is guaranteed to work by the supplier.

If you look in the Apple store, choose a Mac Mini, you'll see an option to have it shipped with 8gb of ram. I have one with 8gb of ram from OWC. Not a problem.

I'm using two 4gb sticks of NuRAM. http://eshop.macsales.com/item/Newer%20Technology/8566DDR3S8GP/ I can't believe how inexpensive these are getting.

Hope you get that freeze sorted out.
Matt

Almost any linux live disk includes a memory test on it.
Try downloading (say) ubuntu live for your mac and boot to the cd.
Then choose the memtest option and let it run.
QED