Code Execution - works on one page and not on another -- same code

Hi,
I'll try to get this right, but it's a bit involved.

I've built a billing database for my company (I work for an ISP).
The database lists customers, their bill rate, their commitment level (in MB or GB) and a few other things.
I have a function that calculates a customer's 95th percentile usage (from a perl script) for the month and places that value into a field -- this peice is working correctly

Based on this result, I then make some calculations for each customer based on their bill rate, commit level, etc. and print a dollar amount in another field.

Problem: If I search on a single customer everything works fine.
If I list more than one customer per page (default 10 when the user logs in) it prints the bandwidth utilization properly (result from the perl file) but does not print the bill calculation.

What confuses me is that these two views are using the exact same code to do the calculation.
I thought that maybe is was a max_execution time setting in php.ini but that is set to 600 seconds -- plenty of time.
Also, as noted before -- the perl results are returning correctly -- it's the internal (no external scripts) function results that aren't printing.

Here is my calculation within php:
   $unit = $field_4;
   $rate = $field_5;
   $commitment = $field_6;
   $excess = $field_7;
   $excessrate = $field_8;
   $billrate = "";
        if ($field_2) {
                if ( $unit == "Mb") {
                        $ninetyfive = ninetyfive($field_2);
                        echo $ninetyfive;
                } else {
                        echo "";
                }
                }
// Begin Calculation for EOM bill rate)
                if ($ninetyfive) {
                        if ($rate > 0) {
                                if ($commitment > 0 ) {
                                        if ( $ninetyfive > $commitment ) {
                                                $over = ceil(( $ninetyfive - $commitment ) / $excess) * $excess;
                                                $billrate = ( $over * $excessrate / $excess);
                                        } else {
                                                $billrate = "";
                                        }
                                }
                        }
                }
   // End Calculation for bill rate (ALMOST) -> see echo statement below
        }
   echo "</font></td>";
        // Print Bill rate line
        if ($billrate) {
                echo "<td align=\"center\" nowrap><font size=\"2\" color=\"green\"><b>$$billrate.00</b></font></td>";
        } else {
                        echo "<td align=\"center\" nowrap><font size=\"1\" color=\"$font_color\"></font></td>";
                }