I don't have an answer for you.
You and a few others have mentioned the problem, but I've never been able to replicate it anywhere where I can do debugging to find out what's causing it.
I think I recall seeing two things that may have given some relief:
a) turn off SSL (ie: ENABLE_SSL set to false) ... do the problems persist?
and others tried:
b) instead of clicking the PayPal button on the shopping cart page, click the regular "Checkout" button. Login and select shipping, and then on the payment page choose PayPal *there*. Then checkout goes fine.
At least, that's what they claimed.



