How to go about tracking down this error?

Not sure I really understand this... so you have:

//Handle +IPD input data from ESP8266
function ipdHandler(line) {
  var colon = line.indexOf(":");
  if (colon<0) return line; // not enough data here at the moment
  var parms = line.substring(5,colon).split(",");
  parms[1] = 0|parms[1];
  var len = line.length-(colon+1);
  if (len>=parms[1]) {
   // we have everything
   sockData[parms[0]] += line.substr(colon+1,parms[1]);
   /** INSTRUMENTED =>**/ console.debug('ipdHandler (empty): ', line.length, ' bytes in packet, ', sockData[parms[0]].length, ' bytes in total');
   return line.substr(colon+parms[1]+1); // return anything else
  } else { 
   // still some to get
   sockData[parms[0]] += line.substr(colon+1,len);
   ///** INSTRUMENTED =>**/ console.debug('ipdHandler (more) : ', line.length, ' bytes in packet, ', sockData[parms[0]].length, ' bytes in total');
   return "+IPD,"+parms[0]+","+(parms[1]-len)+":"; // return IPD so we get called next time
  }
}

So it should be reporting back every single time it gets more data in?

But it actually reports:

ipdHandler (empty):  61  bytes in packet,  591  bytes in total
ipdHandler (empty):  63  bytes in packet,  2051  bytes in total
ipdHandler (empty):  64  bytes in packet,  3511  bytes in total
ipdHandler (empty):  65  bytes in packet,  4971  bytes in total
ipdHandler (empty):  67  bytes in packet,  6431  bytes in total
recv [sckt:0] (deltaTime:  949377358.046 ) :  64  bytes of  7063  sent to socketServer
ipdHandler (empty):  67  bytes in packet,  7827  bytes in total
ipdHandler (empty):  69  bytes in packet,  9287  bytes in total
ipdHandler (empty):  70  bytes in packet,  10747  bytes in total
ipdHandler (empty):  72  bytes in packet,  12207  bytes in total
ipdHandler (empty):  72  bytes in packet,  13667  bytes in total
ipdHandler (empty):  73  bytes in packet,  14632  bytes in total

I'd have expected bytes in total to increment by a bit less than 'bytes in packet' each time, but it's going up by about 1500 bytes!

I think one solution would be to change the netRecv function such that it used JsVars rather than a fixed size buffer. That way it'd read as much data as the network connection was willing to give it.