Error running payload rewrite script

Ok, we have progression, but am still having problems.

I now manage to tweak and get the script running intermittantly. I'm getting a strange HTTP:payload error/quirk. The HTTP_RESPONSE and HTTP_RESPONSE_DATA sections are as follows:

when HTTP_RESPONSE {
   set clen [HTTP::header Content-Length] 
   if { not [info exists clen] or "0" eq $clen} { 
     set clen 4294967295
   } 
  log local0. "HTTP_RESPONSE CLEN= $clen -End"
  if { $clen > 0 } {
    HTTP::collect $clen
  }
}
when HTTP_RESPONSE_DATA {
  set find "findstring" 
  set replace "replacestring"
  set payload "[HTTP::payload]"
log local0. "Payload [HTTP::payload length] [string length "[HTTP::payload]"] [string length $payload]"
   Run the regsub to make all the replacements (add -nocase for case insensitivity)
  if {[regsub -all $find $payload $replace new_response] > 0} {
    HTTP::payload replace 0 [HTTP::payload length] $new_response
  }
}

I'm sometimes getting in the log:

Jan 18 11:55:42 tmm tmm[671]: Rule payload_https_testportal : HTTP_RESPONSE CLEN= 16175 -End
Jan 18 11:55:42 tmm tmm[671]: Rule payload_https_testportal : Payload 16175 16175 16175

Which is correct. But I'm often getting an incorrect:

Jan 18 11:50:16 tmm tmm[671]: Rule payload_https_testportal : HTTP_RESPONSE CLEN= 16175 -End
Jan 18 11:50:16 tmm tmm[671]: Rule payload_https_testportal : Payload 16175 115 115

which means ny regsub doesnt work.

Why does [HTTP::payload length] and [string length "[HTTP::payload]"] differ?? Is this a memory issue?