real time smartphone camera processing v2

I have this code:

url = 'http://192.168.1.108:8080/shot.jpg'; % single frame source
cam = imread(url); % read source

% Parameters to play with
WINDOW_SECONDS = 4;
BPM_L = 40; BPM_H = 230;        % [bpm] Valid heart rate range
FILTER_STABILIZATION_TIME = 1;  % [s] Filter startup transient
CUT_START_SECONDS = 0;          % [s] Initial signal period to cut off
FINE_TUNING_FREQ_INCREMENT = 1; % [bpm] Separation between test tones for smoothing
fps = 20;                       % frame per second
i = 1;
y = [];

%first loop, get 5 seconds of data
while(size(y,2) ~= 100)
    cam = imread(url);         % read source
    pic(:,:,i) = cam(:, :, 1); % take RED domain only
    y(i) = sum(sum(pic(:,:,i))) / (size(cam, 1) * size(cam, 2));
    i = i + 1
    disp('Collecting data...');
end

% butterworth filter
[b, a] = butter(2, [(((BPM_L)/60)/fps*2) (((BPM_H)/60)/fps*2)]);
% 5-points moving average for smoothing
yfilt = filter(b, a, y);
% cuts the first second and takes the remaing for seconds of data
% The first second is the filter transient
y = yfilt((fps * max(FILTER_STABILIZATION_TIME, CUT_START_SECONDS))+1:size(yfilt, 2));
% Some initializations and precalculations
fcl = BPM_L / 60;
fch = BPM_H / 60;
bpm = [];
bpm_smooth = [];

% second loop
% step 1) make calculations on the first 4 seconds of data
% step 2) take the last 3 seconds
% step 3) add 20 new frames, that makes the array 4 seconds long again
% step 4) goto step 1)
while(1)
    if(i==101)    
        % *********************** STEP 1 ***********************
        for j=1:size(y,2)

              gain = abs(fft(y));
              il = floor(fcl * (size(y, 2) / fps))+1;
              ih = ceil(fch * (size(y, 2) / fps))+1;
              index_range = il:ih;
              [pks, locs] = findpeaks(gain(index_range));
              [max_peak_v, max_peak_i] = max(pks);
              max_f_index = index_range(locs(max_peak_i));
              bpm(j) = (max_f_index-1) * (fps / size(y, 2)) * 60;
              freq_resolution = 1 / WINDOW_SECONDS;
              lowf = bpm(j) / 60 - 0.5 * freq_resolution;
              freq_inc = FINE_TUNING_FREQ_INCREMENT / 60;
              test_freqs = round(freq_resolution / freq_inc);
              power = zeros(1, test_freqs);
              freqs = (0:test_freqs-1) * freq_inc + lowf;

              for h = 1:test_freqs,
                  re = 0; im = 0;
                  for k = 0:(size(y, 2) - 1),
                      phi = 2 * pi * freqs(h) * (j / fps);
                      re = re + y(k+1) * cos(phi);
                      im = im + y(k+1) * sin(phi);
                  end
                  power(h) = re * re + im * im;
              end

              [max_peak_v, max_peak_i] = max(power);
              bpm_smooth(j) = 60*freqs(max_peak_i);
          % ******************************************************

          end

          disp(['Mean HR: ' num2str(mean(bpm_smooth)) ' bpm']);
          disp('.');
          % *** STEP 2 ***
          y = y(20:80);
          i = 60;
          % **************  
      end

      % ************************ STEP 3 ***************************
      i = i + 1;
      cam = imread(url);
      pic(:,:,i) = cam(:, :, 1); % take RED domain only
      y(i) = sum(sum(pic(:,:,i))) / (size(cam, 1) * size(cam, 2));
      % ***********************************************************
  end

pastebin version for nicer view http://pastebin.com/tYq7D4mp

This code does the following:

• read frames from webcam source (20 / sec)
• calculate BPM
• do it again in a neverending loop always with the new data that comes meantime

It does not work like I wanted.. first sec 105bpm, next second 60. It fluctuates badly. How should I correct it?