I have encountered mysterious time-to-time "Invalid HTTP Response" errors on slow links.
The problem was socket read() timeout in Transport/HTTP.php
I think reporting timeouts as own soap fault helps in debugging SOAP-based applications.
I propose next patch for Transport/HTTP.php :

--- HTTP.php.b  2003-01-23 12:19:53.000000000 +0200
+++ HTTP.php    2003-01-23 12:25:57.000000000 +0200
@@ -307,9 +307,15 @@
         // get reponse
         // XXX time consumer

-        while ($data = fread($fp, 32768)) {
-            $this->incoming_payload .= $data;
-        }
+        do {
+            $data=fread($fp,4096);
+            $_tmp_status=socket_get_status($fp);
+            if ($_tmp_status["timed_out"]) {
+                return $this->_raiseSoapFault("Timed out read from $host");
+            } else {
+                $this->incoming_payload.=$data;
+            }
+        } while ( !$_tmp_status["eof"] );

         fclose($fp);