Holding down CTRL (sprint) and W (forward) at the same time while in a minecart that is NOT at full speed causes pulses instead of a consistent sprint. 

I experienced this in survival, and recreated it in creative. The best way I found to reproduce it was to make a 24x5 rail ring, with three powered rail at one end. This causes lower speeds throughout the rest of the ring, contributing to the pulsing.