POST data getting lost: Content Length = 0

I've been trying to find a solution for ages with no success. On our app, customers submit a form and on the next page we process it. Sometimes though the data never arrives. This seems to happen for just a few of our customers. Seems to happen with IE7 and using a proxy.

Here's some headers, note the HTTP_VIA:

X-REWRITE-URL: /process.asp?r=699743 
APPL_MD_PATH: /LM/W3SVC/31555/ROOT
APPL_PHYSICAL_PATH: C:\inetpub\vhosts\mysite.com\httpdocs\
AUTH_PASSWORD: 
AUTH_TYPE: 
AUTH_USER: 
CERT_COOKIE: 
CERT_FLAGS: 
CERT_ISSUER: 
CERT_KEYSIZE: 
CERT_SECRETKEYSIZE: 
CERT_SERIALNUMBER: 
CERT_SERVER_ISSUER: 
CERT_SERVER_SUBJECT: 
CERT_SUBJECT: 
CONTENT_LENGTH: 0
CONTENT_TYPE: application/x-www-form-urlencoded
GATEWAY_INTERFACE: CGI/1.1
HTTPS: off
HTTPS_KEYSIZE: 
HTTPS_SECRETKEYSIZE: 
HTTPS_SERVER_ISSUER: 
HTTPS_SERVER_SUBJECT: 
INSTANCE_ID: 31555
INSTANCE_META_PATH: /LM/W3SVC/31555
LOCAL_ADDR: XXX.XXX.XXX.XXX
LOGON_USER: 
PATH_INFO: /process.asp
PATH_TRANSLATED: C:\inetpub\vhosts\mysite.com\httpdocs\process.asp
QUERY_STRING: r=699743
REMOTE_ADDR: YYY.YYY.YYY.YYY
REMOTE_HOST: YYY.YYY.YYY.YYY
REMOTE_USER: 
REQUEST_METHOD: POST
SCRIPT_NAME: /process.asp
SERVER_NAME: www.mysite.com
SERVER_PORT: 80
SERVER_PORT_SECURE: 0
SERVER_PROTOCOL: HTTP/1.1
SERVER_SOFTWARE: Microsoft-IIS/7.0
URL: /process.asp
HTTP_CONNECTION: Keep-Alive
HTTP_PRAGMA: no-cache
HTTP_VIA: 1.1 WEBCACHE-2
HTTP_CONTENT_LENGTH: 0
HTTP_CONTENT_TYPE: application/x-www-form-urlencoded
HTTP_ACCEPT: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/x-shockwave-flash, application/x-ms-application, application/x-ms-xbap, application/vnd.ms-xpsdocument, application/xaml+xml, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*
HTTP_ACCEPT_LANGUAGE: en-gb
HTTP_COOKIE: ASPSESSIONIDQCKSDCTS=FENMPCMDCHEOENGOJPGDGPLN;
HTTP_HOST: www.mysite.com
HTTP_REFERER: http://www.mysite.com/theform.asp
HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727; .NET CLR 1.1.4322; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022)
HTTP_UA_CPU: x86
HTTP_X_REWRITE_URL: /process.asp?r=699743

We've been having the same problem.

POST requests from IE7/IE8 that go through ISA proxy are sometimes (maybe one time in 50) missing the POST data, and have a content-length of 0.

This is a known issue with ISA - see http://support.microsoft.com/kb/942638 which provides a fix.