tachometer:: speed become zero during work

HI 

i have a a tachometer that conenected to motor shaft  

the output of this tachometer is pulses and each 24 pulse give a 1 revoluation

i use labview FPGA to estimate the pulse frequency  with LV VI that already provided by NI as shown in figure (1) the loop tested under 5MHZ

the problem is shown in figure (2)  where during DC rotating the pulse frequency go to zero for then back again to normal.

this cause a problem in my speed control system.

best regards