Managed to resolve this with the help of tech support and solution is below in case others find it useful.
Reverse DNS lookups were indeed the issue. When creating the splunk forwarder input, the default "connection_host" (inputs.conf) is to use reverse dns. In our case no dns/access via firewall was possible to multiple timouts/net traffic generated which was stopping other clients connecting reliably.
Edited the input to "connection_host" to "none", restarted splunk and all was well with no further connection timeouts in splunkd.log etc.
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