Contribution hangs between contribution page and Paypal

We had this problem with our previous (1point something) installation but understood it was planned to be fixed in 2.2.2. Unfortunately, it's still happening to us.

For about 8% of donors the process falls over between filling in the contribution form and passing the info to Paypal. A Pending Contribution is recorded for the donor, which is never completed with a confirmation from Paypal.

The donor knows it isn't working - some of them try several times before we have the chance to contact them. Most recently 1 June.

We use Paypal Express and are using the same contribution pages we wrote in the previous version of CiviCRM.

Thanks.