That's the general conception but spinning fans only mean they can work with low available power. The PSU may be delivering insufficient power to work the Graphics Card which btw are known to consume a good deal of the supplied power. The obvious question; Is the old card lower power than the 7850?.. my gues is it is... and if so, your results would be expected. One thing is for sure, any of both PSU or GPU is the cause.. so testing both is the only way to know.