Não sei qual versão do Apache você está usando. Eu tive um problema semelhante com o Apache 2.2.22, e a documentação do Apache sugere o seguinte:
proxy-initial-not-pooled
If this variable is set no pooled connection will be reused if the client connection is an initial connection. This avoids the "proxy: error reading status line from remote server" error message caused by the race condition that the backend server closed the pooled connection after the connection check by the proxy and before data sent by the proxy reached the backend. It has to be kept in mind that setting this variable downgrades performance, especially with HTTP/1.0 clients.
Mas não tenho certeza se isso resolverá seu problema.