no wonder your card does not run properly, the 250 watt is not near enough. According to Nvidia, you need at least a 300 watt power supply. But I would recommend a 400 or even 500 watt power supply in case you then want to upgrade from the 750ti. Some respectable and well reputed brands are Corsair, Seasonic, OCZ, Thermaltake and an upcoming brand that is getting a lot of good praise are the Sentey power supplies(I have one and man is it great, just a personal experience). But find one that fits your budget and necessity, pick whichever you feel best. One that works perfectly for my computer may not be the one for you.
Here are a few suggestions...