Parallel Download Embedded Resources causes connection timeouts in JMeter test

I have a JMeter test setup to run 250 concurrent users across 20 pages with a uniform random delay of 100 to 5000 milliseconds. These page views run for a set number of loops.

When I run without Retrieve All Embedded Resources or with Parallel Downloads set to 1 the tests work great and everything is fast. When I set parallel downloads to a higher number like 6 for browser emulation I eventually start getting Connection Reset on my sockets.

I can see some TCP reset packets on my network with a tcp dump but I'm not sure if they are coming from JMeter or somewhere in the network.

What would cause the network to reject the parallel connections from JMeter? In the app server we are only seeing a few connections at a time. Does JMeter do anything special for those parallel connections as it relates to the HTTP socket?

We've tried with both the HTTP4 and Java connection type and can't figure out why the parallel download is causing issues.

The error we get is this one coupled with some connection reset errors:

Thread Name: Thread Group 1-10
Sample Start: 2018-06-14 11:45:47 CEST
Load time: 1956
Connect Time: 1956
Latency: 0
Size in bytes: 2497
Sent bytes:0
Headers size in bytes: 0
Body size in bytes: 2497
Sample Count: 1
Error Count: 1
Data type ("text"|"bin"|""): text
Response code: Non HTTP response code: java.net.SocketException
Response message: Non HTTP response message: Socket closed