We currently do many IPN transactions per day. We've been very happy with the "Belt and Suspenders" method described as Option 4 PDT+IPN here: http://web4us.wordpress....paypal-ipn-integration/ However, we are missing 4 or 5 transactions per week. While the transactions are a very small amount of the total transactions, handling the same 4-5 support requests every week is tedious and makes for unhappy customers.

My best guess for what's happening is that our server is either down very briefly or too busy and failing to respond to the POST from paypal.

What should I do to trouble shoot the problem?
Is this rare or does this happen to others?
What steps can I take to reduce the frequency of missing payments?

Again, it works almost perfectly for many, many, many transactions.