Power Supply is bad Ron.
Electrolytic Capacitors inside the Power Supply are breaking down.
Since you didn't state the model number, I'll just choose one for an example, and the Troubleshooting Tools and Utilities information;
Took you the 'Long way' to show you the steps in finding troubleshooting information. May help you in the future.
"Blinking Yellow - A power supply or system board failure has occurred"
Now about Electrolytic Capacitors;
The Electrolytic Capacitors used on your motherboard, (Unless some are solid Polymer capacitors), are Radial Aluminum Electrolytic Capacitors.
A basic example,
Basic construction of a Radial Aluminum Electrolytic Capacitor:
A) The case is a thin aluminum cylindrical shape. Looks like an aluminum 'Coca-Cola' can with no top, or bottom.
Wrapped with a plastic sleeve.
B) The top is a flat thin aluminum round disk, and has a shape etched partway into it. Usually a lK or X.
This is the Vent Cover.
C) The bottom is a flat synthetic rubber disk, and is called the Bung.
The two leads of the capacitor come through the Bung.
D) Inside the case of the capacitor are three strips;
1) One strip is thin aluminum metal, and is called the Conducting Strip. It has the Positive ( + ) lead attached to it. (Lead - think wire)
2) The second strip is also thin aluminum metal, but has a non-conducting medium applied to it.
It is the Non-Conducting Strip, and has the Negative lead attached to it.
3) The last strip is a paper-like strip, and is soaked with Electrolytic Paste.
The paper soaked strip is laid in-between the two metal strips, and all three are rolled up tightly.
(In reality there are many layers of these strips)
When an Electrolytic Capacitor starts to fail, the Electrolytic Paste develops a gas inside the capacitor. Hydrogen Gas.
The gas expands, and starts pushing Electrolytic Paste out of the capacitor.
Either the lK or X of the Vent Cover will pop open, and paste will ooze out,
Paste will ooze out of the seal area, around the outside edge of the Vent Cover,
One side of the Bung at the bottom of the capacitor will be pushed out, and paste will ooze out.
So much paste loss, and the capacitor operates at a weakened state.
TOO much paste loss, and the capacitor fails.
A capacitor is designed to slowly build up a charge, then release it all at once.
Older camera designs use capacitors for their flash unit.
Another way of explaining, although crude, is a large swimming pool is slowly filled with water by a garden hose, then one wall of the pool is taken down all at once.
This is why a Power Supply can start to work, then fail.
Some of the capacitors inside are failing.
Just thought it would be better than stating, "Replace your Power Supply"
Do I recommend trying to repair the Power Supply?
Dangerous (As in could KILL you), voltages are inside, even with the Power Supply unplugged from power, and for weeks, months, and over a year, after being unplugged from power.
Also clean the inside of the computer out with a can, or two of compressed air for computers. Computer unplugged from power, AND Anti-Static Precautions -> FOLLOWED.
The Power Supply in your computer is an SMPS.
Switched-Mode Power Supply
The two cooling components for the SMPS are Heatsinks, and a small internal Fan,http://en.wikipedia.org/wiki/File:ATX_power_supply_interior-1000px_transparent.png
The Heatsink/s are shown in-between the letters B and C, and
C and D.
Finned aluminum 'blocks' connected together in a vertical line.
(Vertical line in the photo)
A Heatsink absorbs heat from whatever object it is placed against, and the fins of the Heatsink radiate the heat away.
The small internal fan helps carry heat away from the fins.
When in-between the fins, and around the fins of the Heatsink, are clogged with 'Gunk', and the Fan's blades, center hub, and surrounding cage (Shroud), are clogged with 'Gunk', the cooling capacity drops tremendously.
Just takes a light coat of 'Gunk'.
('Gunk' = Dirt, dust, hair, lint, etc)
Heat = Wasted Energy
The Power Supply tries to keep up with the call for power, but eventually cannot, and components inside start to fail.
For additional questions please post in a Comment.
[One more thing. Dell used to be a proprietary computer manufacturer. This means they wanted THEIR parts used in THEIR computers.
One design change they had made, was to change the standard pinout of the ATX main power cable, coming from the Power Supply.
Had the Power Supply manufacturer design, and make Power Supply's to their (Dell) ATX main power cable designation.
They also had the motherboard manufacturer, change the ATX main power cable connector pinout, on the motherboard.
Examples of both types of ATX main power cables that are used,
1) 20-pin ATX main power cable,
2) 24-pin ATX main power cable,http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24
1) Orange wires are 3.3 Volts. (DC)
2) Red wires are 5 Volts. (DC)
3) Yellow wires are 12 Volts. (DC)
ALL Black wires are Ground wires.
You can see the Standard pinouts for the wires above in the links.
USING the Lock on the side of the ATX main power cable's connector, use it to orient the connector, and follow the pinout of those wires.
Are they the same as the ones in the links?
People with these older types of Dell computers in the past, were replacing the Power Supply, BUT not buying from Dell.
This means they bought aftermarket Power Supply's.
Wiring pinout for ATX main power cable did Not match.
MANY people burned out their computers.
'Smoked' the motherboard, Processor, Ram Memory, Harddrive, and a graphics card IF used.
Yes. People were SO happy! (?)
Dell changed pretty quickly.
Check your ATX main power cable.