Unstable instances and unexpected instance shutdown with GAE / java

Question

I'm running a java app and I suffering constant instance shutdown under GAE. I use a b8 backend with manual scaling, just 1 instance (the instance is resident) The pattern of restart is as follows: if there is no traffic instance will last more than 6 hours. As soon as there is some normal traffic, instance is shutdown in about 20 minutes average. There is no entry in the log file nor ah/stop invocation. I'm discussing this issue with GAE support but there is no final resolution yet Is it normal instances just will live 20 minutes and then they commit suicide without any logged error? Is any one suffering from this pattern? Thanks