1. Standard is less reliable than Express Checkout. Standard was designed originally for very simple sites that aren't really "stores" ... that is, stores that are NOT database-driven like Zen Cart.
It has to wait for PayPal's server to tell *your* server about the purchase ... because once the customer leaves your site to go to PayPal's site (when using Standard), your site knows NOTHING about what happens next, even if your customer is sent back to your store from PayPal's site ..... until PayPal's servers tell your store (in a completely separate background process) about the payment.
However, Express Checkout sticks with the customer all the way through the process, even going to PayPal's site and back again. Thus the entire transaction is WAY more reliable. It's built on much newer technology than Standard was ever designed for.
Thus, it's a much better decision to use EC than to use Standard. And since the experience for the customer is the same in the end, there's really not much difference in the end result, except that your store actually gets the completed orders immediately.
But, if you want to continue trying to fix your Standard problem, see the hundred steps here:
http://www.zen-cart.com/wiki/index.php/PayPal