iRule causing ever-increasing TMM CPU utilization

I've adapted an iRule written by Deb Allen to rate limit (in count, not in bps) PDF downloads from one of our web applications. It basically counts the number of PDF's downloaded in the past 10 seconds, and gives an error response if they are exceeding the threshold. The rule works as expected. However, as the rule is in place, TMM CPU utilization slowly increases, until performance is affected after about a week. If I re-initialize the iRule (make a small modification and save, or simply remove and reassign to the virtual) the TMM utilization drops back down to normal levels. I recently upgraded from 9.2.3 to 9.4.4 in hopes that a couple memory leak issues were the culprit, but the issue remains. iRule timing shows that the avg CPU time for the iRule steadily increases as time goes on. I'm thinking the issue is due to the array growing in size, despite the lines clearing the older entries out. Is there a way to automate periodically re-initializing the array? Below is the iRule:

 rule pdfapp-abuse-detection-rule
 Limits the rate of PDF downloads for a particular IP client
 Adapted from 'RateLimit_HTTP' on devcentral 
 Adapted by Michael Rintzler
 August 20th, 2007
 Original concept by Deb Allen, F5 Networks
 April 2006
 
when RULE_INIT {
  set ::pdfappmaxRate 10          ;set later per user from class
  set ::pdfappwindowSecs 10           ;global
  init array if non-existent
  array set ::pdfHistory { }
  wipe array if already existent
  array unset ::pdfHistory
}
when HTTP_REQUEST timing on {
  if { [HTTP::method] eq "GET" } {
    if { ([HTTP::uri] contains ".pdf") and not ([HTTP::header exists Range ]) and ([HTTP::cookie exists ERIGHTS])} {
      log local0. "pdfapp PDF Initial download detected."
      Extract clients IP address
      set client_ip [IP::remote_addr]
      set pdfapp_session [HTTP::cookie ERIGHTS]
      set currentTime [clock seconds]
      we need to count requests in last $windowSecs seconds, so mark the cutoff time 
      set pdfappwindowStart [expr {$currentTime - $::pdfappwindowSecs}]
      find GETs for this sessionID
      set pdfCount 1
      set mypdfappMaxRate $::pdfappmaxRate
      count GETs within the window, delete those that are older
      foreach { requestID requestTime } [array get ::pdfHistory ${pdfapp_session}*] {
        count pdf downloadss with start time > $pdfappwindowStart, delete the rest
        if { $requestTime > $pdfappwindowStart } {
          incr pdfCount 1
        } else {
          unset ::pdfHistory($requestID)
        }
      }
       if { $pdfCount < $mypdfappMaxRate } {
        Allow request and add new record to array w/myUserID.rand + currentTime
 set pdfapprequestID to a random number
        set requestID "${pdfapp_session}.[expr { int(10000000 * rand()) }]"
        set ::pdfHistory($requestID) $currentTime
        log local0. "PDF Abuse - download from $client_ip, SESSION $pdfapp_session - Current rate: $pdfCount" 
     } else {
        Reject request with 200 response
        log local0. "PDF Abuse - User $client_ip, Session $pdfapp_session"
        return
      }
    }
      else { log "Partial PDF download detected." }
  }
}