Question SMC using encoder creeps

My rig uses encoders on the motors and I'm trying to use SMC.
I've got an UNO, IBT-2 and a small 12v motor with reduction gearbox and an osepp encoder fitted to motor shaft set up for testing.
I've got the encoder into the SMC code and:
-the SMC monitor displays a near perfect overlay of target and feedback
- the motor appears to be in sync but is advancing (or regressing).
It did not behave like this using a pot for feedback so I don't think the gears are at fault.
Instead of keeping a constant sweep of 2 > 5, 5 > 2, 2 > 5,,, it goes something like 2 > 5, 5 > 2, 2 > 6, 6 > 3, 3 > 7,,,
the encoder is mapped:
SWmulti = map(SMPulseCount, -512, 512, 0, 1023);

How may I stop creeping?

thanks

and here's the sketch I'm inserting, I can't post the SMC sketch with it in it (too many characters)

Code:

int SMsensor01;
int SMsensor02;
int SWmulti;
int SMstepOld;
int SMstep;
long SMPulseCount;
#define SMSENSOR_A 2
#define SMSENSOR_B 3

void checkState(){
  SMsensor01 = digitalRead(SMSENSOR_A);
  SMsensor02 = digitalRead(SMSENSOR_B);

  if(SMsensor01 == 1 && SMsensor02 == 1){
    SMstep = 0;
    if(SMstepOld == 1){
      SMPulseCount--;
    }
    if(SMstepOld == 3){
      SMPulseCount++;
    }
    SMstepOld = 0;
  }

  if(SMsensor01 == 0 && SMsensor02 == 1){
    SMstep = 1;
    if(SMstepOld == 2){
      SMPulseCount--;
    }
    if(SMstepOld == 0){
      SMPulseCount++;
    }
    SMstepOld = 1;
  }

  if(SMsensor01 == 0 && SMsensor02 == 0){
    SMstep = 2;
    if(SMstepOld == 3){
      SMPulseCount--;
    }
    if(SMstepOld == 1){
      SMPulseCount++;
    }
    SMstepOld = 2;
  }

  if(SMsensor01 == 1 && SMsensor02 == 0){
    SMstep = 3;
    if(SMstepOld == 0){
      SMPulseCount--;
    }
    if(SMstepOld == 2){
      SMPulseCount++;
    }
    SMstepOld = 3;
  }

}

void setup() {

 


  attachInterrupt(0, checkState, CHANGE);
  attachInterrupt(1, checkState, CHANGE);
  //Serial.begin(9600);
  SMPulseCount = 0;
}

void loop() {

 
 
  if(SMPulseCount > -1){
   
  }

  if(SMPulseCount < 10 && SMPulseCount > -10){

  }

  if(SMPulseCount < 100 && SMPulseCount > -100){

  }


  if(SMsensor01 == 1){

  }
  else{

  }



  if(SMsensor02 == 1){

  }
  else{

  }

  SWmulti = (SMPulseCount - 850);
  SWmulti = constrain(SWmulti, 0, 45);
 
    Serial.print("SMPulseCount: ");
    Serial.print(SMPulseCount);
    Serial.print("\t");
    Serial.print("SWmulti: ");
    Serial.println(SWmulti);
    //Serial.print("SMPulseCount: ");
    //Serial.println(SMPulseCount);
}