You can read the specifications of your card on the following link : ASUS Ultimate GeForce GTX 550 Ti Video Card Review - The ASUS Ultimate GeForce GTX550-Ti - Legit Reviews
"The ASUS Ultimate GeForce GTX 550 Ti 1GB video card requires a 400 Watt or greater power supply with a minimum of 24 Amps on the +12 volt rail. It also requires that the power supply has one 6-pin PCI Express power connector for proper connection.
It should be noted that the NVIDIA minimum system power requirement is based on a PC configured with an Intel Core i7 3.2GHz CPU. If you want to run SLI we have been told that a 500 Watt or greater power supply is suggested by NVIDIA. NVIDIA refuses to give +12V requirements for SLI, which is strange since they suggest them for single card solutions. Notice that ASUS has the power header for their GeForce GTX 550 Ti comes off the top corner of the video card rather than the end of the card. This is great for those with small computer cases."
That is the point! maybe your PSU has enough power to start the card, but once you launch an app that requires more graphics, your GTX 550 fails due to a weak PSU!
That's just my opinion, maybe I'm wrong, but i gave it a try
Fill in your detail system specs!