Well if your psu only goes to 18A, its a miracle already that it works.
Thats prob also the reason why its not stable @stock.
Get a decent psu, an el cheapo class can take your whole setup with it to electricheaven when it goes out with a bang.
Good brands are Corsair, PC and Powercooling, Seasonic, Thermaltake.
Read here for good reviews about psu's:http://www.jonnyguru.com/
And be sure to get a single rail with enough amps ont the +12V line.
A minimum of 600W, ~50A on +12V is required according to google/nvidia.