If it runs, but cuts out after a certain amount of time, the thermal shutdown circuits of the PS must be kicking in. That usually means it is being pushed too hard. No matter how many resistors you use on the 5v or 3.3v line won't fix that; overloaded is overloaded.
The current PS you are using may just be too small on the 12v line to handle what you are trying to do. Look at the label on the side of the supply and read the +12v spec. Pick the supply with the highest current value.
If you are seriously having a hard time getting it to work, you can ship the best one to me and I can see what I can do to make it more stable. Don't know if the two-way shipping cost would be worth it to you, and there is no guarantee if I can do anything to make it better, but it's an option. Just PM me if that sounds like something you want to try. Seems a shame to go through several PS units and none work like they should...
BTW: Make sure you take that other green wire (the one that goes to the 120v AC ground plug) and connect it to the case somehow. That is to protect you from getting shocked should the case (being metal) somehow float at 120v. If that were to happen, a circuit breaker would blow, but that's better than electrocution.
