Late to the party here and of no practical use except to testify occasional but annoying replications of exactly the same as the OP: (Motu 4Pre firewire on a TI chipset PCIe with latest 64-bit drivers, W7Pro64(SP1), Sonar x3 Producer 64bit(d),.
My present personal experience is that I'm more likely to trigger the FW interface's thowing-in of its towel by periods of cycling through Dimension Pro progs. I've no idea if this of universal significance.
I've presently got a headscratching list of other odd behaviours(now including my own as well as those of the trio of X3, W7, and the Motu
)...thus, my present, though head-in-a-bucket "workaround", is plenty of saves, close X3 when the Motu quits, turn off Motu, turn on Motu, fire up X3 and rock+roll.
(Turning off my firewire device whilst Sonar is running, even when X3 informs me my device is no longer available, then turning the FW on again, has no effect).
I have little knowledge and even less time, so as to presently allow me to do an Enterprise-Picardian level 3 diagnostic of all variables in W7, X3 and my Motu FW...I'm merely chipping in to say that a) I do feel your pain, as my experience tallies with yours; b) my feeling is that this is not an RME-specific error; c)my wild, wild and supernatural guess is that there is still some default W7 desire to attempt to wrestle back default audio control from a firewire device to the onboard audio...but of course this doesn't explain random behaviour(in my case) of soundcard hari-kiri whenever I get crazy with Dimentia Pro.