I have a QE4 and have had no such problems.
I do know that it does not tolerate much lint accumulating in the bobbin area. I am 'good' about cleaning it after a good run for the day. I use a sable brush that is made for children. Thin plastic handle.
Anyway, I would be on the phone with the people that sold you the machine and have already brought your machine back to for repair. If possible go and talk to them face to face. Ask for other contact numbers if they are unable to resolve the problem or answer your questions. I would also try to get in contact with the main Pfaff office in the USA. You have been having this problem when your machine it was still under warrantee.
Keep us posted as to the outcome, please.