I cannot help you with everything. However I have found this the most successful sendAndLoad application:
Your use of myVars for your reply could cause problems.


Code:
{		myVars = new LoadVars();
		MyReply= new LoadVars();
		myVars.username = userName.text
		myVars.pass = userPassword.text
		myVars.action = 'login';
		MyVars.sendAndLoad('your.php, MyReply);
		MyReply.onLoad = UpdateReply;           ///don't put brackets '()'
}
function UpdateReply(success) {
	if (success) {
		_root.Vareply = MyReply;
		
	} else {
		_root.Vareply = "Failed";
		
	}
}