"Is your new power supply rated lower that your previous one? What is the wattage? Note that the quality (and price) also affect a PSU's ability to deliver (it's never 100%). Above 400 watts should be fine though.
I had the same problem when running a 200watt GTX260 on a 430watt power supply (never had a problem with a 7900GT). I realize a 7600GT will not nearly take that much wattage (around 50watts in 3D), but it could be a sign that your PSU is borderline or poorly builded. My old PSU emittied a slight whining sound whenever it was in 3D mode, showing it was being put to the limit.