Is there an application to test for bad memory?

My 2,8 Mac pro is very unstable and I have tried reinstalling the whole shebang with 10.5.6, 10.5.7 and 10.5.8
I have also tried shuffling the Magma card for my chassis.

I suspect the reason for my frequent crashes is bad memory. Reason for this is first time I upgraded my ram only 2gb of my 10gb got recognized and after I swapped the memory riser all 10gb got recognized (I have 2x 1gb on one riser and 2x4gb on the other).
So I took out the two 1gb dimms and put each 4gb dimm in the first slot of each riser. On first boot only 4gb got recognized and after swapping the risers again all 8gb got recognized.
When only 4gb where recognized my system didn't crash and after swapping the risers where it recognized the full 8gb it did crash.
Because I wanted to test the 'riser configuration' where only 4gb got recognized I swapped the risers again and now it also recognizes the full 8gb.
This is kinda concerning, so my suspect is that one of the memory risers is somewhat faulty or that one of my DIMM's us somewhat faulty.

I was wondering if someone else had similar experiences and if there is a good memory testing application around?