Studio XPS 435mt new PSU

I've been having issues with my XPS 435mt turning on for several months now. It started where the computer would only turn on after being left off for a day or two. When it wouldn't turn on the power button would flash amber whenever you pushed it

Eventually it 'turn on at all anymore (the blinking amber light was still present), so I disconnected the PSU from everything it was connected to, and then reconnected it and the computer worked fine for about 4 days, turning on and off without any trouble. Eventually it stopped turning on again, and I tried disconnecting and reconnecting it again, but this time it still wouldn't turn on, and the power light wouldn't blink anymore.

I went to buy a new, cheap psu, it had a 20 +4 pin, and my mobo has a 24 pin and an 8 pin. When I connected the new psu, the power light now stays solid amber, but still will not turn on. I'm wondering if I'm maybe not connecting the psu correctly, do I connect the 20 +4 pins to the 24 pin slot on the mobo? or do I connect 20 to the 24 slot, and the +4 to the 8 pin slot?

Any help is appreciated here.