not enought juice at the starter

sometimes when i start my 97 taurus it gives just a little less juice at the starter than it needs to throw the solenoid its just a hair under what she needs. ive replaced the battery .cables, starter, neutral switch, motor mounts, what could cause just a little loss of juice every ten or twenty starts. very intermittant. thank you

Possible you have ground trouble. Cleaning battery cables/terminals, adding a few more of your own wont hurt either, from block to frame and body, and run em back to the neg terminal never hurts.