Ah... ok. So Elton, if that is indeed your real name ;-p... one other thing, and you may have mentioned this already, is that there was known issue with some of the ASUS P87XXXX MOBOs. I own one (I forget the exact model right now but I think it is in the same immediate family as yours) and they made a very specific point of saying that I MUST download and install the latest BIOS from their site because the version it shipped with was problematic. So perhaps that's what's going on (it would fit the behavior you are describing). So before you ditch the Onyx or do something else drastic you may want to look into that. Now keep in mind updating the BIOS is kind of a big deal so back up all your stuff and be prepared to do a full reinstall anyway. I updated my BIOS before I installed anything else so I didn't run into any problems but I have had people mention that it caused them some serious grief when trying to slap it on to an already configured system loaded with programs. While you're at it you might as well snag all the other drivers from their site and put them on disc. Not necessarily install them ALL (there are some things like Bluetooth adapter drivers and crap like that likely aren't necessary for a DAW and perhaps even harmful due to resource drainage) but keep them handy. For example I had to download and install USB3 drivers for my board (not sure why but I did). Do NOT however install the ASUS Utility Suite. It's useless and is extremely difficult to remove. There are much better resource monitors out there that do NOT behave like bloatware.
That said... WinXP and Sonar 8.5 were both very different animals than Win7/8 and Sonar X series. If the Mackie is no longer being supported I'm guessing they half assed the drivers in later versions. If you still encounter problems or want to be future ready then I'd definitely start looking at new interfaces. Considering you do have one of the good Firewire cards then that opens a few more door and in fact, with the right interface, you may be better off using Firewire. There has been speculation that Firewire is on the way out (and I'm guilty of feeding into that speculation) but I'm hearing more and more now that for audio and video at least Firewire is sticking around. Just REALLY do your research into any issues with any device you are considering. Again Focusrite has the Saffire series interfaces which are supposedly good and well priced and I've seen a lot of people talk about the RME stuff (Fireface I think is the series name).
Still, USB works fine and is a lot easier to move from machine to machine without all these extra considerations as well as having a bit more choice and as far as I can tell cost less if you go with the right brand.
Again... I am not a tech. Just an observer. Hopefully you find a solution to your woes. Cheers.