Paypal shopping cart problems

[dingdong05]

This isn't for ebay really, although I guessed someone here may use paypal for another webshop.
I do, and have spent quite a while creating and applying buttons for all my products. Occasionally one or 2 will stop working, but now most of them have screwed up for no reason!
In stead of going to a shopping cart it takes you to the sign in page.
I nearly lost one customer, who knows how many have tried, failed and then just walked away?

Anyone else had this problem?