Problem with IR tachometer

Using Arduino Project Guidance
1

I have written a code but my RPM reading from my IR tachometer keeps on fluctuating.
An example is shown below and the motor is constantly spinning around 990RPM
00:57:01.278 -> =====================
00:57:01.278 -> RPM : 1194
00:57:01.583 -> =====================
00:57:01.583 -> RPM : 995
00:57:01.854 -> =====================
00:57:01.888 -> RPM : 995
00:57:02.157 -> =====================
00:57:02.190 -> RPM : 796
00:57:02.457 -> =====================
00:57:02.491 -> RPM : 995
00:57:02.794 -> =====================
00:57:02.794 -> RPM : 1593
00:57:03.064 -> =====================
00:57:03.099 -> RPM : 1194
00:57:03.368 -> =====================
00:57:03.401 -> RPM : 1593
00:57:03.669 -> =====================
00:57:03.703 -> RPM : 1194
00:57:03.971 -> =====================
00:57:04.004 -> RPM : 995

Attached is the code i used

// Pin Allocation
#define trigPin 10      // Ultrasonic Sensor Trigger Pin
#define echoPin 11      // Ultrasonic Sensor Echo Pin

float rev=0;
int rpm;
long time=0,duration, distance ,oldtime=0;
void isr()                                    // Interrupt Service Routine to count the pulse od the IR Sensor
{
if(digitalRead(2)==LOW)
  rev++;
}

void setup()
{
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);                   // Ultrasonic Trig Pin as Output
  pinMode(echoPin, INPUT);                    // Ultrasonic echo Pin as Input
  attachInterrupt(0,isr,FALLING);             // Attaching the interrupt to interrupt pin0(Digital Pin 2) with Falling Edge
}

void loop()
{
// --------------------------------- Tachometer Reading using IR ------------------------------------------------
  detachInterrupt(0);                         // Detaches the interrupt
  time=micros()-oldtime;                      // Finds the time
  Serial.println("=====================");
  //Serial.println(rev);                      // Print the Revolutions on Serial monitor
  rpm=(rev/time)*60000000;                       // Equation to calculates RPM
  //Serial.println(oldtime);                     // Print the time on Serial monitor
  //Serial.println(time);                     // Print the time on Serial monitor
  Serial.print("RPM        : ");
  Serial.println(rpm);                        // Print the RPM on Serial monitor
  oldtime=micros();                           // saves the current time
  rev=0;
  attachInterrupt(0,isr,FALLING);              // Attaching the interrupt to interrupt pin0(Digital Pin 2) with rising Edge 
 
  

  delay (300);
   

}
2 
void isr()                                    // Interrupt Service Routine to count the pulse od the IR Sensor
{
if(digitalRead(2)==LOW)
  rev++;
}

Your interrupt is triggered by a falling edge on pin 2.

Some questions:
Why are you checking pin 2 to see if it is low?
Why are you using a float value for rev?
Why isn't rev qualified as volatile?
Why are you attaching and detaching the interrupt?

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