yup your specs are fine, you shouldn't have any issues system related unless a hardware failure somewhere.
firewire is on its way out the door, just keep this in mind.
I know your device is more than capable but keep in mind this unit was released over 10 years ago.
how old is the unit [ Mackie Onyx 1620 with firewire ] ?
this may be having issues.
its a far shot but it may also have something to do with a windows update.
if your problem didn't exist to long ago or you have a back up. I would try restoring it.
this would narrow down your research for the problem.
theres also another fail safe way to illuminate your device as the problem.
download Asio4all.
http://www.asio4all.com/ if you have no problem when using asio4all, this will tell you its the unit.
or something along the stream.