Applescript freezes upon first run when getting 'text of document 1 of application "Safari" '

Question

I'm working on an applescript that searches the text of a Safari web page for specific text.  The script  freezes / hangs / times out ONLY during the "first run".  By "first run"  I mean the first time the script is run after a period of time has passed. If the script is run a 2nd, 3rd, or 4th time, etc soon thereafter it works perfectly.  If you then wait an extended period of time again, the script freezes exactly as if it were a "first run".  This appears to be independent of whether Safari is open or closed, i.e. the script freezes even if Safari remains open between the last working run and the "first run".
Details:
A web page is opened.  I subsequently confirm the page is fully loaded with the following code set-up as a handler:
tell application "Safari"
     repeat with i from 1 to 15
     if (do JavaScript "document.readyState" in current tab of window 1) is "complete" then
                         exit repeat -- page loading is complete, exiting loop     
     else
                         delay 1
     end if
     end repeat
     if i is greater than or equal to 15 then return false
     if i is less than 15 then return true
end tell
The Applescript event log confirms loading is complete and returns a value of true.  I can see the page is fully loaded, and in testing it is fully functional. All is good.
I then search the text of the fully loaded web page for specific text.  It is this next step where Applescript hangs, but ONLY on the "first run":
set searchTerm to "find this"
if text of document 1 of application "Safari" contains (searchTerm as string) then
     -- do stuff
end if
The event log shows that applescript starts to "get every text of document 1" and even returns the initial text of the web page but hangs.  Applescrpt returns the following error: "Safari got an error: AppleEvent timed out.  Error -1712".  If I rerun the script at this point the full text of the web page is immediately displayed in the event log, and the script continues succesfully to the end.  The "IF" statement is not the issue per se because I get the same error if I remove the IF and instead set a variable to the value of 'text of document 1 of application "Safari".
Bottom line - Applescript seems to hang while returning the text of the fully loaded web page, but ONLY on "first run". Any ideas?
Note:  I've run this directly as an applescript and as a service via Automator.  Same result. I also tried putting the computer to sleep briefly, wake and rerunning the script.  This does not repeoduce the problem so while extended gaps in use cuases the problem, it does not appear that going to sleep during this time is sufficient to cause the issue.

Answer 1

Thank you twtwtw, very helpful!
I tried fiddling around with the name of the input variable(s): $@, $0, and $1, and although I could get it to work somewhat, the hangup was always on high ASCII characters like …, —, °, etc…  (They were called “high ASCII” in the old days, perhaps they are called something else now.)
Checking the long list of output encodings via iconv --list, and trying a bunch of them didn’t work for me.
But the real trick that worked for me was to do as you said and to change the “Pass input” menu to “to stdin.”
However, using -t ASCII was suboptimal for high ASCII, so I changed it to -t UTF-8-MAC and it works perfectly now, all the wild international characters and punctuation are maintained! Hooray!  However, upon further testing, removing the pipe to iconv, it seems it’s not needed.
So this is the final command:
awk 'BEGIN{print "<ul>"} {print"<li>"$0"</li>"} END{print "</ul>"}'
So for now this is the whole recipe to create Contextual Menu Items for the Services menu that replaces text snippets with HTML code surrounding them.
1. Create a new Automator Service document.
2. Check the box “Output replaces selected text.”
3. Choose Actions, under Library choose Utilities, then drag Run Shell Script to the working area of the window.
4. Set the Pass input menu to “to stdin.”
5. Paste in the command (above).
6. Save it; I prefix my HTML CMIs with HTML (space space), so this one is, “HTML  unordered list.”
7. Begin using it. :-)
Since it took me a while to cobble together the other ones I made, I should probably share them.  Once I get the time to bring over the other “involved” ones I’ll see if I can find out where is the best place to do that.
Thank you both, red_menace, and twtwtw for your help!