Restarting project and getting closer :) Need a little help with an Interrupt

A little while ago, I was working on a digital tach for my car, but had a big problem with the displays flickering. The flickering is fixed, but now the RPM is completely wrong [noparse]:([/noparse] I think it has to do with the interrupt messing with the PULSIN commands. A few members gave me advice to use the COUNT command, but i could never get it precise. I was also given a few other alternatives for the PULSIN command, but i could not get them to work correctly.

Here is what i have so far :

DEVICE          SX28, OSC4MHZ, TURBO, STACKX, OPTIONX
FREQ            4_000_000
ID              "M-SEG"

RPM_signal    PIN     RC.0 INPUT
TRIS_disp    VAR     TRIS_B
rpm        VAR     WORD
pWidth0        VAR     WORD
pWidth1        VAR     WORD
dividendMSW     VAR     WORD
dividendLSW    VAR     WORD
overflow    VAR     BIT
doneBit        VAR     BIT
display        VAR     RB
digit        VAR    WORD
digit1        VAR    WORD
currentrpm    VAR    WORD

INTERRUPT 120
  digit = currentrpm
  LOW RA.1
  digit1 = digit / 100  ' Get First digit
  digit = __REMAINDER
  READ SegMap + digit, display
  pause 5
  HIGH RA.1
  LOW RA.2
  digit1 = digit1 / 10  ' Get second digit
  digit = __REMAINDER
  READ SegMap + digit, display
  pause 5
  HIGH RA.2
  LOW RA.3
  digit1 = digit1 / 10 ' Get Third digit
  digit = __REMAINDER
  READ SegMap + digit, display
  pause 5
  HIGH RA.3
RETURNINT


PROGRAM Start

Start:
  TRIS_disp = %00000000

Main:
    PULSIN RPM_signal, 0, pWidth0
    PULSIN RPM_signal, 1, pWidth1
    pWidth0 = pWidth0 + pWidth1
    pWidth0 = pWidth0 * 2
    dividendMSW = $005B
    dividendLSW = $8D80
    rpm = 1
    overflow = 0  
    DO
      doneBit = rpm.15
      rpm = rpm << 1
      IF overflow = 1 THEN
        rpm.0 = 1
        dividendMSW = dividendMSW - pWidth0
      ELSE
        IF dividendMSW >= pWidth0 THEN
          rpm.0 = 1
          dividendMSW = dividendMSW - pWidth0
        ENDIF
      ENDIF
      overflow = dividendMSW.15
      dividendMSW = dividendMSW << 1
      dividendMSW.0 = dividendLSW.15
      dividendLSW = dividendLSW << 1
    LOOP UNTIL doneBit = 1
    rpm = rpm << 1
    rpm.0 = overflow
    IF dividendMSW >= pWidth0 THEN
      rpm.0 = 1
    ENDIF
    ' Set variable currentrpm to prevent the interrupt from getting var rpm while in the middle of calculations
    currentrpm = rpm + 150
  GOTO Main

SegMap:                                         ' segments maps
  DATA  %00111111                               ' 0
  DATA  %00000110                               ' 1
  DATA  %01011011                               ' 2
  DATA  %01001111                               ' 3
  DATA  %01100110                               ' 4
  DATA  %01101101                               ' 5
  DATA  %01111101                               ' 6
  DATA  %00000111                               ' 7
  DATA  %01111111                               ' 8
  DATA  %01100111                               ' 9

I am only using 3 LED displays because that is all the would fit on my test board so I just want to read the first 3 digits. When I hook it up to my car and let it idle, the display jumps from 2000+ rpm to 500 rpm even though it is idling at 800 rpm. I was trying to find if there was a way to call an interrupt from inside the code only when needed and figured that may fix my problem, but I have not been able to find anything like that. I am wanting to only use the internal clock because I want to keep the circuitry as simple as possible.

Any help is greatly appreciated [noparse]:)[/noparse]