1:1 ratio is the full speed of the memory module, when its run as DDR or double data rate, your end produced rate is twice the SIMM. To my understanding, if its not running 1:1 stable, its not running its advertised speed. If I am in-correct please someone advise me. I cannot explain this further cause of my absence of further knowledge on this. Personally I don't think it changes even with DDR3 as the 3 is just a sub-variation or progression or the Double Data rate. You don't have 3 sides of a single line which is the physical shape of a ram stick only 2 sides.
If you didn't test it at 1:1 when you first got it and its past it's warranty, you may be out of luck on a return. I do have a stable (2x2) 4g kit of DDR2 1000Mhz...