well you're using 1x 256MB with what timings on DFI ? can you show your a64tweaker settings ? mines 2x 256MB so need looser timings at 2-2-2-6-8-12-2-2-1-2

also since memory bandwidth plays a part newcastle bandwidth is higher than clawhammer cpus clock for clock

3400+ newcastle memtest reports 1427MB/s at 200mhz 2-2-2-6
3700+ clawhammer memtest reports 1326MB/s at 200mhz 2-2-2-6