There are two factors that can help with this sort of problem. One is connecting power directly from the supply to a couple of other points on the chain, in this case the end and the middle in addition to the beginning might help a lot. The second factor is supply voltage, 9V is the maximum for ShiftBrites but that doesn't mean you should try to run it that high if you have a choice. For LEDs, lower voltage is better because less power needs to be thrown away as heat. If possible, try using a 6V supply. Someone else reported very good results with a 5V supply, though that is below recommended voltage due to the internal regulator.