reseting variable - what can produce in program execute

[phoenix_1]

At first look when my electronic work and in practice all is ok.
But I must know if I can find what it is?
If I have next variables in program:

setpoint var word

And when I reset as :

     select case setpoint                         
     case is >= 65534                             
     setpoint = 0                                   ; reset setpoint
     POSCNTH = 0 
     POSCNTL = 0 
     end select

At reset condition from 65534 to 0 case very very low shake in PID function on my controller.I know that becouse if I write next "case is >= 6250 "
case more frequently that thing.That extra small shake produced on hpwm -CCP1.
Here is full test code.
PMDC_Pid.txt