Timing problem ?

Klaus, your remark confirm my findings:

Having replaced the assignments

FA_errno = Constants.FE_Success;

with

InitFA_errno ();

led to a correct behaviour of the script. The original logic is from Russ Ward at https://forums.adobe.com/thread/895626. BTW the small function is:

function InitFA_errno() {
// Reset FA_errno as it is write protected
// see https://forums.adobe.com/thread/962910
  app.GetNamedMenu("!MakerMainMenu");            //If this fails, you've got bigger problems
}

Before I had the idea to replace the assignment of the FA_errno I tried the 'various methods offered on the net' for setting a sleep timer. These all created a hanging FM, although cpu usage was very moderate.

So now the comple function FindAndReplacePara is this

function FindAndReplacePara (targetDoc, findString, sourceDoc, trReplace, loopMax) {
// ----------------------------------------------- Replace string by textrange (part of pgf)
// FindText, ConsiderCase; Function returns replacemtCounter
// Since we have collected only in doc.MainFlowInDoc, we replace only in this flow
// loopMax used as emergency back door - twice the # of initially found citations
// We do not have an initial TextSelection, hence start directly at the first para
// Source: script by Russ Ward in https://https://forums.adobe.com/thread/895626; 2081392
//        and Rick Quatro
  var tr = new TextRange();
  var restoreTR, frame = 0, loopCounter = 0, replacementCounter = 0;
  var findParams = new PropVals();
  var firstPgf = targetDoc.MainFlowInDoc.FirstTextFrameInFlow.FirstPgf;

  tr.beg.obj = tr.end.obj = firstPgf;            //  set up the starting text range as the very beginning
  tr.beg.offset = tr.end.offset = 0;              // of the flow. We'll move straight from beginning to end.
  trSaved = tr                                    // to come back after work
//Set up find parameters. We want to configure it to look for a string and perhaps be case sensitive.
//We don't need the find to wrap because we are controlling the flow from beginning to end.
  findParams = GetFindParams (findString);        // Get the find parameters for finding string, case sensitive

//  FA_errno = Constants.FE_Success;              // errno global, to be used to track the progress of the find and replace
  InitFA_errno ();                                // reset - it is write protected
  tr = targetDoc.Find(tr.beg, findParams);        // and do an initial find to get started.
                                                  // find and replace loop as long as we keep finding
  while(FA_errno === Constants.FE_Success && loopCounter++ < 2*loopMax) { 
    targetDoc.TextSelection = tr;                // set up the text range to clear the original text
    targetDoc.Clear(0);                          // clear it
    app.ActiveDoc = sourceDoc;                    // switch to sourceDoc
    sourceDoc.TextSelection = trReplace;          // to be able to get the replacement text
    sourceDoc.Copy(0);
    app.ActiveDoc = targetDoc;                    // switch to targetDoc
    targetDoc.Paste(0);                          // and paste the replacement text
    ReApplyFontAndSize (targetDoc, tr);            // re-apply the current para format

//  FA_errno = Constants.FE_Success;              // ...  find the next instance. We'll reset FA_errno again just in case
    InitFA_errno ();                              // reset - it is write protected
    tr = targetDoc.Find(tr.beg, findParams);      // something screwy happened while we were replacing text.
  }
  targetDoc.ScrollToText(trSaved);                // we're done. Restore the document to it's original area of display
  replacementCounter = loopCounter;
  return replacementCounter;
} // --- end FindAndReplacePara