HttpWebRequests using WebProxy work and then fail after time
After much digging - I went back to the proxy provider again with a bunch of trace information and digging results and... miraculously... they've decided that they do in fact have limitations that they have not disclosed to the purchaser.
Shocker. I'm more annoyed I went this far after initially discussing with them and getting a clean bill a health on their end.
If anyone needs a recommendation for a provider not to use :)