the psu has a mobo attached to let the psu run.
No need for that. Connect green wire from the ATX 24-pin motherboard connector to any black wire.
Heck, at work I even have a super special custom paper clip I use for powering up and testing suspect computer power supplies.
Check the ripple and noise of your PSU.
I'd check this too. An all too common problem with computer power supplies is that the capacitors go bad, causing excessive ripple which then causes the computer to randomly crash or not boot up at all. Could be what's messing up your DDS.
Or.... it could just be a faulty piece of eBay garbage.