The PayPal Sessions Viewer lets you push the order into the database if no other methods are working.
If your problems are sporadic, then it's likely a delay happening from PayPal's end.
You could try turning on the PayPal debugging to see whether there are any specific failures.
You said sporadic ... but are there any patterns / similarities amongst the "failed" notices? ie: certain currency, certain price range, etc?
Have you tried refunding 0.01 to re-trigger the order notification to Zen Cart?



