If your working Proxy server eventually fails with either of the subject messages, and refuses to access any more pages, unless you stop and start the service, your page file may be too small.
For Proxy to work effectively, I have found that the page file should be at least RAM + 32meg.
Use Control Panel / System / Performance / Virtual Memory / Change to set the Initial and Maximum page file size the same and at least 20 megabytes higher than the recommended size. Restart your server.
NOTE: You could receive the same messages if you don't reinstall your latest service pack after installing Proxy.