Hi,
I have had this problem in the past with G5 DL360s and DL380s.
I use Samsung PC2-5300 DDR2-667MHz ECC Dimms in my systems and they work fine, so have stuck with these from now.
Part number is
M395T5160QZ4-CE68 for the 4GB dimms
M395T5663QZ4-CE68 for the 2GB dimms
Hope this helps
Cheers
Leon