SSL needs quite a lot of spare memory

src/net/tcp.c

 		start_timer ( &tcp->timer );
 
 	/* Estimate window size */
-	window = freemem;
+	window = ( ( freemem * 3 ) / 4 );
 	if ( window > TCP_MAX_WINDOW_SIZE )
 		window = TCP_MAX_WINDOW_SIZE;
 	window &= ~0x03; /* Keep everything dword-aligned */