Forms Pro Issue - lose responses on submission

Hello

I have switched back to Forms Basic after experiencing an issue with data loss, using Forms Pro. I sent out a quiz with 100 questions in a Forms Pro. When submitted, out of 130 responses, 15 Forms lost all their data (the forms are blank , form responses were not recorded on the server), some saved just the first response. 

Retaking the quiz; same computer, same browser doesn't always replicate the issue.

I hope someone can look into this bug ASAP.

Thanks!