"export audio" error with external FX

Using Cubase 6 with the MR816csx interface and XP pro.

Has anyone else had an error when trying to export audio with external effects patched in?

Can’t remember exactly what the error said, but after the export starts, it pops up and stops the export, stating something about “cpu overload”, although when viewing the cpu in Cubase while exporting shows no overloads.

Again, this only happens when external hardware is patched in, and the export is taking place in “real” time. Eventually it will export, but obviously this shouldn’t be happening.

Are there any fixes?