What is the maximum DPC latency reported over a few minute time span? And go ahead and open your DAW application to see if any spiking occurs under that condition. I'm sure there are TONS of other factors to consider, but every PC I've built has performed well assuming the DPC's are in check (below 100us, and preferably around 10-20us steady).
Beyond that, Firewire adds to the complexity. IIRC, I've read that using the legacy Firewire driver tends to work much better with RME gear than the newer W7 Firewire driver. You also have the FW chipset itself to consider, but if it was working fine under XP, then the FW chipset itself shouldn?t be an issue. I'm not much of a Firewire guy, so I'll let others speak on that...
FWIW ? I know iTunes 64 causes MASSIVE DPC spikes when downloading Podcasts ? just for your info :-)
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