Playback stops unless I'm on Internal Sync

X1
Windows 7 64
THe symptom is : hit PLAY and I get 1/2 second of playback then stop....literally a blip.
 
I originally was using MAudio Delta RBus PCI, outta the blue  I got no audio at all .   WOrked for the longest thyme then poof! nuttin.
I reloaded Sonar, tried a million different buffer settings, share the driver or not, checked boxes that were unchecked, unchecke boxes that were checked, WDM, ASIO, swingin' dead chickens overhead while chanting....nuttin.
 
Although.....I did get it to run passed the 1/2 second & continue when I clicked Internal Sync, but had no Audio.....weird.
 
So I figured the Not-Intended-For-Windows7-and-especially-not-64-bit Gods had FINALLY gotten around to PUNISHING me for using a 1990's erra PCI card.   So off Craigs' list I got a Saffire (6In/10out.....old one) firewire.
 
Now I get playback and  audio output when I"m on the Internal Sync, but it immediately flips back to Audio Sync. ..... and from there it does the 1/2 second blip & stops. 
 
So the difference is (from my readings) whether the Audio clock comes from the audio device or from the motherboard of the PC.  I can't imagine what's blocking the stupid audio clock thru the PCI slot (my firewire interface is PCI)....for that matter, I wonder if the clock is REALLY coming from the PC and THEN slips back to Audio? Or if it flips to Audio when immediately after PLAY with initial setting of Internal????   
 
Heppp me....I've fallen & I can't get Audio....

I do not own the Saffire but I believe it is the same as most other devices in that it either syncs internally or to a s/pdif input (digital input on the Saffire).  Unless you are using the s/pdif inputs to connect to another piece of equipment you want to set your clock to internal. I have no Idea why it is changing but my guess would be that the reason it is stopping is that it is not receiving a clock signal from the s/pdif when it changes from internal. I would go online and download a manual if you don't have one and double check the setup in the mix control.

Internal can only be used for MIDI (no Audio) and as you've found out if the project has audio it will auto switch.
 
Are you sure it's a clock issue and not something else that is causing a drop out? Have you updated any drivers lately? Sometime updates cause problems, I've had to skip a version of drivers for my interface (A&H ZED-R16) for something similar.
 
Other issues might be I/O buffers or even something with the project, unless it's all projects of course.