Sometimes loosening the timings is better, and clocking higher - specifically on Eth.
I've seen you mention loosening the CAS timings. I tried bumping up tCL by 1, but still get crashes on the K4G4 at 2100. So is it just loosening tCL that usually does the trick, or something else too?
You have to loosen it on the DRAM, too - you're loosening the tCL on the ASIC, but not the DRAM, throwing them off.
Weren't the straps that control the dram settings precisely? You have to change values somewhere else?