I think that it could be related to threading and http.... maybe I'm wrong but let me explain:
Subscriber is using a couple of threads to call to the both services. Probably is reusing a existing one thread to call to service1 and creating a new one to call to service2 (no idea, just a guess). What about if http is expecting the response in the thread1 but the response is arriving in the thread2???? when response arrives in thread 1, works! when response arrives in thread 2.... fails!
just an Idea...