As long as the delay between shots is the same you should get very consistent drops. When photographing just a single drop it should appear in the same place every time.
Using TIP122’s and 2.2k ohm resistors shouldn’t make too much difference, if any, as the slowest part of the process is the valve. But you can try with a smaller resistor, around 150 ohm, and see what happens. If you don’t have 150 ohm try something similar, like 180, 220, 330, etc.
If not already, it may be worth while checking the insides of the valves. Make sure the spring is working correctly. I have just added a couple of videos showing the insides of the valves and how to clean them: http://www.dropcontroller.com/solenoid-valves/