One possible cause of this is that ports are being reused too frequently from the child proxy. If this is the cause of the problem confirm the following settings on the child proxy:
#(config) tcp-ip inet-lowport 16384
#(config) tcp-ip tcp-randomize-port enable
These settings will enable 48K source ports to be used instead of the default 16K. It is always good to have tcp randomization enabled for security reasons.