Paypal Express Timeout

I have been having trouble in the last 2 or 3 months with this. The user is timed out at step 3 of the checkout process even though this is only a few seconds after step 2.
I have checked with my ISP [webhosting.uk.com] and they assure me the server is fine. I am waiting for Paypal to get back to me but I am not expecting anything positive from them.
I have logs and info files related to 2 transactions if that helps. Is there any mod I need to make to Zen config?

1. Does this happen to all customers, all the time?
2. Can you make it happen yourself by doing a transaction yourself?
3. When you click "reply" below to give your answers, there's a "Posting Tips" section. Please include answers to all those questions too.

I have had Zen ver 1.3.8a for many months. Thinking about it, the problem may have started when paypal was modded to do away with the verisign entries. I am sure it worked before that. I am having the same problem on at least 2 websites that I run.
This is an intermittent fault - I have made a dummy purchase and it worked fine.
ISP is webhosting.uk.com who say all is ok with server.
There does not appear to be an answer to this problem in the forum or faq.
I have also stopped receiving confirmation emails from Paypal - but only for SOME of the orders.
Thanks in advance for your help.