I don't have any solutions either I'm afraid, but maybe it might help to try and work out why. With mine, sometimes (but not always) when she's on the lead and we come across a strange dog all her hackles go up and she howls, but at least I know it's about fear, so I try to act as though nothing scary is happening and say hi to the other dog's owner etc. I know in her case it's all about fear because off lead we never have the problem (or she would never go off that lead!), she just gives the other dog a wide berth and acts submissive, and I'm pretty sure that the on lead problem dates back to one particular incident with a dog that was aggressive towards her when she was on the lead and couldn't run away.