HTTPClient intermittently locking up server - asp.net-core

Question

I have a .NET Core 2.2 app which has a controller acting as a proxy to my APIs.
JS makes a fetch to the proxy, proxy forwards call onto API's and returns response.
I am experiencing intermittent lock ups on the proxy app when its awaiting the response from the HttpClient. When this happens it locks up the entire server. No more requests will be processed.
According to the logs of the API that is being proxied to it is returning fine.
To reproduce this is i have to make 100+ requests in a loop on the client through the proxy. Then i have to reload the page multiple times, reloading it whilst the 100 requests are in flight. It usually takes around 5 hits before things start slowing down.
The proxy will lock up waiting for an awaited request to resolve. Sometimes it comes back after a 4 - 5 second delay, other times after a minuet. Most of the time i haven't waited longer then 10 min before giving up and killing the proxy.
I've distilled the code down to the following block that will reproduce the issue.
I believe im following best practices, its async all the way down, im using IHttpClientFactory to enable sharing of HttpClient instances, im implementing using where i believe it is required.
The implementation was based on this: https://github.com/aspnet/AspLabs/tree/master/src/Proxy
I'm hoping im making a rather obvious mistake that others with more experience can pin point!
Any help would be greatly appreciated.
namespace Controllers
{
[Route("/proxy")]
public class ProxyController : Controller
{
private readonly IHttpClientFactory _factory;
public ProxyController(IHttpClientFactory factory)
{
_factory = factory ?? throw new ArgumentNullException(nameof(factory));
}
[HttpGet]
[Route("api")]
async public Task ProxyApi(CancellationToken requestAborted)
{
// Build API specific URI
var uri = new Uri("");
// Get headers frpm request
var headers = Request.Headers.ToDictionary(x => x.Key, y => y.Value);
headers.Add(HeaderNames.Authorization, $"Bearer {await HttpContext.GetTokenAsync("access_token")}");
// Build proxy request method. This is within a service
var message = new HttpRequestMessage();
foreach(var header in headers) {
message.Headers.Add(header.Key, header.Value.ToArray());
}
message.RequestUri = uri;
message.Headers.Host = uri.Authority;
message.Method = new HttpMethod(Request.Method);
requestAborted.ThrowIfCancellationRequested();
// Generate client and issue request
using(message)
using(var client = _factory.CreateClient())
// **Always hangs here when it does hang**
using(var result = await client.SendAsync(message, requestAborted).ConfigureAwait(false))
{
// Appy data from request onto response - Again this is within a service
Response.StatusCode = (int)result.StatusCode;
foreach (var header in result.Headers)
{
Response.Headers[header.Key] = header.Value.ToArray();
}
// SendAsync removes chunking from the response. This removes the header so it doesn't expect a chunked response.
Response.Headers.Remove("transfer-encoding");
requestAborted.ThrowIfCancellationRequested();
using (var responseStream = await result.Content.ReadAsStreamAsync())
{
await responseStream.CopyToAsync(responseStream, 81920);
}
}
}
}
}
EDIT
So modified the code to remove the usings and return the proxied response directly as a string instead of streaming and still getting the same issues.
When running netstat i do see a lot of logs for the url of the proxied API.
4 rows mention the IP of the API being proxied to, probably about another 20 rows mentions the IP of the proxy site. Those numbers dont seem odd to me but i don't have much experience using netstat (first time ive ever fired it up).
Also i have left the proxy running for about 20 min. Its it technically still alive. Responses are coming back. Just taking a very long time between the API being proxied to returning data and the HttpClient resolving. However it wont service any new requests, they just sit there hanging.